UNIVERSITY   OF    CALIFORNIA 

COLLEGE   OF   AGRICULTURE 

AGRICULTURAL    EXPERIMENT   STATION 

BERKELEY,  CALIFORNIA 


BARK  DISEASES  OF  CITRUS  TREES 
IN  CALIFORNIA 


HOWARD  S.  FAWCETT 


BULLETIN  395 

October,  1925 


A  reprint  in  part  of  Bulletin  360  on  Gum  Diseases  of 

Citrus  Trees  in  California,  with  revisions,  additions 

and  abridgments 


UNIVERSITY  OF  CALIFORNIA  PRINTING  OFFICE 

BERKELEY,  CALIFORNIA 

1925 


Digitized  by  the  Internet  Archive 

in  2012  with  funding  from 

University  of  California,  Davis  Libraries 


http://www.archive.org/details/barkdiseasesofci395fawc 


BARK  DISEASES  OF  CITRUS  TREES 
IN  CALIFORNIA* 


HOWARD  S.  FAWCETT 


INTRODUCTION 

This  publication  is  a  revision  and  abridgment  of  Bulletin  360 
brought  up-to-date  and  supplemented  by  the  addition  of  decorticosis 
(shell  bark)  and  several  other  bark  and  gum  diseases. 

Its  aim  is  to  bring  together  the  information  now  at  hand  regarding 
the  bark  diseases,  including  the  various  major  and  minor  forms  of 
gumming  in  Citrus  trees  under  the  conditions  in  California.  Spe- 
cial emphasis  is  given  to  contributing  conditions  and  to  methods  of 
prevention  and  treatment.  A  considerable  amount  of  investigational 
data  not  heretofore  published  is  included.  The  results  of  investiga- 
tions in  other  phases  of  some  of  these  diseases  have  been  published 
elsewhere.1  These  phases  will  therefore  receive  only  brief  treatment 
here.  It  is  necessary,  however,  to  duplicate  in  part  the  descriptions 
of  the  diseases  and  a  small  number  of  other  data. 

Previous  investigators  had  come  to  the  conclusion  that  gum  dis- 
eases of  Citrus  trees  in  California  originated  independently  of  micro- 
organisms.2 It  was  held  that  these  diseases  were  largely  autogenous 
in  their  nature,  and  frequently  induced  through  the  effects  of  certain 
climatic  or  soil  conditions  alone.  It  now  appears  evident  that  the 
environmental  conditions  cannot  by  themselves  initiate  the  severe 
forms  of  gummosis  in  Citrus  trees  earlier  attributed  to  them,  although 
certain  conditions  are  found  to  play  (as  they  do  in  most  parasitic 
diseases)  an  important  role  as  contributing  factors  which  favor  infec- 
tion and  invasion  of  the  host  by  the  casual  parasites. 

The  investigational  work  on  which  the  greater  part  of  this  bulletin 
is  based  was  begun  in  February,  1912,  under  the  direction  of  the  State 
Commission  of  Horticulture  and  continued  after  October,  1913,  at 
the    Citrus    Experiment    Station    of    the    University    of    California, 


*  Paper  No.  129,  University  of  California,  Graduate  School  of  Tropical  Agri- 
culture and  Citrus  Experiment  Station,  Eiverside,  California. 

i  Fawcett,  H.  S.,  Jour.  Agri.  Ees.  24:191-236.    1923. 

2  Smith,  E.  E.,  and  Butler,  O.  Gum  disease  of  citrus  trees  in  California. 
Calif.  Agr.  Exp.  Sta.  Bull.  200:235-272,  1908. 


4  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Acknowledgment  of  the  assistance  rendered  during  this  investigation 
has  been  given  elsewhere.3 

The  names  of  species  and  varieties  of  Citrus  used  will  be  as 
follows:  common  lemon,  Citrus  limonia  Osbeek;  rough  lemon,  a  horti- 
cultural variety  of  C.  limonia  Osbeek;  sweet  orange,  C.  sinensis 
Osbeek ;  sour  orange,  C.  Aurantium  L. ;  citron,  C.  medica  Linn. ;  tri- 
foliate orange,  Poncirus  trifoliata  Raf .  ;4  grapefruit,5  C.  maxima 
(Burm)  Merrill,  (C.  grand-is  Osbeek).  The  word  lemon,  when  used 
alone  will  refer  to  the  common  lemon,  and  the  word  orange  to  the 
sweet  orange. 


PYTHTACYSTIS    (BROWN  ROT)    GUMMOSIS 

HISTORY 

A  destructive  form  of  gum  disease  similar  to  Pythiacystis  gum- 
mosis first  attracted  serious  attention  in  the  Azores  about  1834.  A 
similar  gum  disease  appeared  in  Italy  as  early  as  1863 ;  in  Portugal, 
1865 ;  in  Australia,  1867 ;  in  Spain,  1871 ;  in  the  United  States,  1875 ; 
and  in  most  other  Citrus  regions  before  the  year  1890.  See  accounts 
and  references  by  Savastano,6  Swingle  and  Webber,7  Butler,8  and 
Fawcett.9 

In  the  early  history  of  Citrus  growing  in  California  there  appears 
to  be  no  record  of  the  occurrence  of  gum  diseases  until  about  1875. 10 
Not  long  afterwards  they  became  an  important  hindrance  to  commer- 
cial Citrus  culture,  as  is  evidenced  by  the  horticultural  literature  of 
the  time. 

A  committee  of  growers  appointed  to  examine  the  condition  of 
Citrus  orchards,  stated  in  187811  that  at  that  time  few  localities 
were  free  from  gum  disease,  which  this  committee  believed  was  caused 


3  Fawcett,  H.  S.,  Two  fungi  as  casual  agents  in  gummosis  of  lemon  trees  in 
California.     Mo.  Bull.  Calif.  State  Comm.  of  Horticulture,  2:601-617,  1913. 

4  Swingle,  W.  T.  il  Citrus, ' '  in  Bailey,  L.  H.,  Standard  Cyclopedia  of  Horti- 
culture, 2:270-785,  New  York,  1914. 

5  Merrill,  E.  D.,  and  Lee,  H.  A.,  A  consideration  of  the  species  Citrus  maxima 
(Burm)    Merrill.   Am.   Jour.   Bot.    11:382-384.      1924. 

g  ' '  Gummosi  degli  agrumi, "  in  Patologia  arborea  applicata  127-141.  Napoli, 
1910. 

7  The  principal  diseases  of  Citrus  fruits  in  Florida.  U.  S.  Dept.  Agr.  Div. 
Veg.  Phys.  &  Path.  Bull.  8:1-42,  1896. 

s  A  Study  on  Gummosis  of  Prunus  and  Citrus,  Ann.  Bot.  25:107-153,  1911. 

9  Gummosis  of  Citrus,  Jour,  of  Agr.  Eesearch.  Vol.  24:  191-236,  1923. 

io  Mills,  J.  W.,  Citrus  fruit  culture.  Calif.  Agr.  Exp.  Station,  Bull. 
138:1-46,  1902. 

ii  Southern  Calif.  Horticulturist,  1:115,  1878. 


BULL.  395]       BARK    DISEASES   OF    CITRUS   TREES    IN    CALIFORNIA  5 

by  excessive  irrigation  and  unsuitable  cultivation.  The  orchards 
most  heavily  flooded  with  irrigation  water,  especially  on  heavy  soils, 
were  found  worst  affected.  The  application  of  manure  immediately 
around  the  trees  was  also  reported  as  tending  to  promote  the  disease. 
Lemon  roots  were  found  to  be  more  susceptible  than  orange  roots, 
and  in  a  discussion  which  followed  this  report  much  objection  is 
expressed  to  lemon  and  Chinese  lemon  stock  as  compared  with  sweet 
orange.  Light  soils  were  thought  by  many  to  be  better  adapted  than 
heavy  soils  for  orange  culture  since  orchards  on  light  soils  were 
healthy  while  those  on  heavy  soils  were  rapidly  dying  out,  presumably 
from  gum  disease. 

Irrigation  by  flooding  was  a  common  practice  at  that  time  and 
later12  it  was  stated  as  a  settled  fact  that  the  lemon  (on  lemon  roots) 
is  peculiarly  sensitive  to  moisture  and  easily  begins  to  rot  if  water  is 
left  standing  around  it.  In  the  following  year  the  statement  was 
made  in  the  same  publication13  that  gum  disease  was  induced  by  mid- 
summer irrigation  and  that  lemon  trees  on  their  own  roots  were 
dying  rapidly.  In  1882  a  leading  nurseryman14  regarded  gum  disease 
as  the  only  trouble  of  importance. 

Of  the  four  Citrus  stocks  commonly  used  at  that  time  (sweet 
orange,  lemon,  lime  and  citron)  the  lemon  and  the  lime  were  stated 
by  Garey  to  be  the  most  fatally  affected  with  this  disease.  He  advises 
the  use  of  the  sweet-orange  stock  but  says  nothing  of  the  sour  orange 
which  was  introduced  at  a  later  date.  Garey 's  description  of  the 
disease  and  his  reference  to  the  differences  in  susceptibility  of  varieties 
indicate  that  the  type  of  disease  spoken  of  was  Pythiacystis  gummosis. 
Because  of  the  suceptibility  of  the  lemon  the  growers  gradually  dis- 
covered that  other  stocks  must  be  used.  The  universal  verdict  accord- 
ing to  Holt15  in  1892  was  that  lemons  should  not  be  grown  on  their 
own  roots. 

Sweet  orange  stock  though  much  less  susceptible  than  lemon  was 
also  frequently  affected  with  gummosis.  This  fact  drew  attention  to 
the  sour-orange  stock  which  had  been  used  successfully  in  southern 
Europe  and  in  Florida  to  replace  trees  affected  with  mal  di  gomma. 
This  resistant  stock  only  gradually  came  into  use  as  a  preventive  of 
gum  disease  in  California.  Cutter16  in  1892  stated  that  his  attention 
was  first  called  to  the  value  of  the  sour-orange  as  superior  to  sweet 


12  Southern   California  Horticulturist,   1:314-315,    1878. 
is  Southern   California  Horticulturist,  2:83-86,   1879. 

14  Garey,  Thomas  A.,  Orange  culture  in  California,  227,  pp,  San  Francisco, 

1882. 

is  Holt,  L.  M.,  "Lemon  culture  for  profit,"  Proc.  Calif.  Pom.  Soc,  1892. 
is  Cutter,  J.  E.,  Proc.  Calif.  Pom.  Soc.  1892 


b  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

in  its  resistance  to  gum  disease  in  1885.  The  following  year  the  firm 
of  Twogood,  Edwards  and  Cutter  of  Riverside,  received  the  first 
shipment  of  sour-orange  trees  from  Florida  for  commercial  purposes; 
although  a  few  trees  had  been  grown  from  Florida  seeds  for  test  by 
others  before  this  time. 

In  the  light  of  our  present  knowledge  of  varietal  susceptibility  of 
Citrus  to  gum  diseases  and  of  the  part  played  by  heavy  soils  and  by 
excessive  irrigation  (especially  flooding  on  heavy  soils),  the  failure 
in  growth  on  lemon  stocks,  and  the  dying  of  trees  on  heavy  soils, 
but  not  on  lighter  soils  would  now  appear  to  have  been  due  to  the 
presence  and  attack  of  gum  disease  organisms,  especially  Pythiacijstis 
citropththora  Smith  and  Smith. 

The  early  discontinuance  of  the  use  of  lemon,  lime  and  citron  as 
stocks,  and  the  adoption  of  the  orange  as  a  general  stock  for  all 
varieties  appears  to  have  been  largely  brought  about  by  this  one 
disease.  Sour-orange  stocks  were  introduced  from  Florida  later,  as 
previously  indicated,  for  the  purpose  of  combating  this  disease  on 
heavy  soils,  but  this  stock  has  not  been  widely  used  in  California 
until  recent  years. 

SYMPTOMS 

Pythiacystis  or  brown  rot  gummosis  with  its  associated  rot  of  the 
fruit17  is  probably  the  most  widespread  and  destructive  of  the  gum 
diseases.  On  the  lemon  (the  most  susceptible  variety)  patches  of 
bark  on  the  trunk  are  killed  and  often  large  quantities  of  gum  are 
exuded  (figs.  1  and  2a).  Infection  usually  starts  at  the  base  of  the 
trunk  or  on  the  crown  roots  and  works  rapidly  upward,  downward, 
and  laterally.  The  bark  is  killed  (not  as  in  cases  of  psorosis  and 
shellbark,  merely  in  the  outer  cortical  layers)  but  entirely  through  to 
the  wood,  thus  including  the  cambium.  A  thin  layer  of  wood  tissue 
only  about  %2  °f  an  incn  thick  is  visibly  affected  unless  secondary 
organisms  enter.  The  bark  above  the  soil  is  not  softened  as  it  is  in 
the  early  stages  of  Botrytis  gummosis  but  remains  firm  and  intact 
until  drying  causes  it  to  shrink  and  crack  longitudinally  (fig.  2b). 
Below  the  surface  of  the  soil  secondary  organisms  frequently  set  up 
fermentation  and  moist  decay.  On  the  bark  of  old  orange  trees  and 
other  partially  resistant  varieties  often  the  progress  of  the  disease  is 
soon  arrested  and  the  lesions  tend  to  become  self  limited.  The  loss  of 
large  patches  of  bark  is  followed  by  a  gradual  yellowing  and  dropping 
of  leaves  on  the  branches  leading  out  from  the  portion  of  the  trunk 
affected. 


17  Smith,  E.  E.,  and  others,  The  brown  rot  of  the  lemon.     Calif.  Agr.  Exp. 
Sta.  Bull.  190:1-72,  1907. 


BULL.  395]       BARK   DISEASES   OF    CITRUS   TREES   IN    CALIFORNIA  7 

Although  these  are  the  main  characteristics  of  the  disease  as  seen 
on  superficial  examination,  there  are  many  special  features  which 
appear  when  more  careful  study  is  given  to  its  development  under 
various  conditions.  In  the  earlier  stages  of  the  disease  the  exuding 
gum  is  usually  the  only  external  symptom  (fig.  2a).  By  lightly 
scraping  the  bark  at  this  time  the  margin  between  the  sound  and 
invaded  tissue  is  shown  indefinitely,  and  only  by  the  gradual  shading 
of  the  normal  green  color  into  a  drab.  The  bark  is  not  softened  but 
remains  firm  and  only  after  a  considerable  time  does  it  shrink  and 
crack  longitudinally  (fig.  2b). 

On  healthy,  rapidly  growing  lemon  trees  the  area  of  killed  and 
darkened  bark,  which  is  elliptical  or  irregular  in  outline,  is  usually  5 
to  10  inches  in  vertical  length  and  half  that  in  width,  when  the  gum 
first  becomes  apparent.  By  this  time  the  fungus  has  been  invading 
the  tissue  usually  for  a  period  of  from  2  to  4  months.  The  removal  of 
the  bark  at  this  time  will  show  that  the  outer  margin  of  the  invaded 
zone  in  the  inner  tissues  is  about  coextensive  wTith  that  seen  on  the 
surface.  The  upward  and  downward  extension  from  the  point  of 
infection  is  usually  many  times  greater  than  the  lateral  extension. 

In  an  irregular  zone  or  band  surrounding  an  actively  invaded 
area,  the  cambium  layer  shows  an  influence  extending  from  the 
margins  of  the  dead  bark.  There  is  a  production  of  clear,  watery 
gum  which  seems  to  originate  in  the  region  of  the  embryonic  wood 
among  the  live  cells  without  any  apparent  fermentation  or  decay. 
This  region,  not  yet  darkened,  outside  the  invaded  portion,  will  be 
spoken  of  in  this  paper  as  the  "outer  gummous  zone"  (fig.  3b).  It 
may  in  time  extend  considerable  distances  upward  and  downward  and 
small  distances  laterally  from  the  margin  of  the  invaded  zone  (fig. 
3b).  It  has  been  traced  for  2  and  3  feet  upward.  The  extent  of  this 
outer  gummous  zone  varies  with  the  age  and  rapidity  of  development 
of  the  disease  lesion,  the  condition  of  the  tree,  etc. 

The  inner  surface  of  the  bark  in  the  invaded  zone  in  a  lesion  of 
considerable  size,  varies  in  color  from  mineral  brown  to  burnt  umber 
or  fawn18  and  the  same  discolorations  will  be  found  on  the  surface  of 
the  wood  just  at  or  beneath  the  cambium  (fig.  3b).  The  discoloration 
does  not  extend  far  (usually  only  Y10  to  %  of  an  inch)  into  the  woody 
layers.  The  cambium  region  in  the  gummous  zone  is  chamois  to 
yellow  ochre  in  color,  gradually  fading  at  the  margins  into  the  normal 
color  of  the  sound  wood}^  surface. 


18  Ridgway,  Robert,  Color  standards  and  color  nomenclature,  43  pp.,  Wash- 
ington, D.  C,  1912. 


8  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Frequently,  when  the  bark  is  irregular  in  contour,  gum  pockets 
from  1  to  2  inches  in  length  will  be  formed.  The  gum  accumulates 
near  the  cambium  and  by  pressure  separates  the  bark  from  the  wood 
at  certain  places,  forming  definite  pockets.  The  pressure  is  usually 
relieved  by  a  break  in  the  bark  before  the  pockets  become  large.  A 
few  deeper  gum  pockets  of  considerable  size  have  also  been  found, 


Fig.  1. — Pythiacystis  or  brown-rot  gummosis  on  lemon  trunk.  The  tree  is 
completely  girdled,  the  dead  bark  resulting  from  several  infections  starting 
near  the  soil  surface.     The  ridges  of  exuded  gum  are  characteristic. 


BULL.  395]       BARK    DISEASES   OF    CITRUS   TREES   IN    CALIFORNIA  J) 

situated  in  the  outer  gummous  zone  beneath  layers  of  wood  Vs  to  % 
of  an  inch  in  thickness,  showing  accumulations  of  gum  under  pressure. 
The  gum,  which  is  watery  and  clear  when  first  formed,  hardens  as  it 
comes  to  the  surface,  apparently  through  loss  of  water,  and  finally 
becomes  brittle.  On  the  surface,  the  hardened  gum  is  usually  mahog- 
any to  chestnut  in  color.18  The  gum  accumulates  on  the  surface  in 
long  narrow  ridges  (figs.  1  and  2a)  or  in  oval  masses,  or  runs  down 
and  collects  in  masses  on  the  soil,  according  to  the  rapidity  of  its 
formation  and  the  dryness  of  the  air.  During  periods  of  heavy 
dews  and  rains  it  gradually  dissolves  and  disappears.  The  invading 
hyphae  frequently  die  out  rapidly  behind  the  marginal  fringe  of 
advance,  and  quite  often  they  die  out  completely  over  a  part  or  all 
of  this  outer  margin,  so  that  the  progress  of  the  disease  is  checked  or 
entirely  arrested.  Such  cases  are  often  found  among  trees  having 
some  power  of  resistance,  especially  among  orange  and  grapefruit 
trees ;  or  when  the  weather  becomes  unfavorable  to  the  parasite  after 
infection. 

In  trees  on  which  the  disease  has  been  present  for  a  long  time,  the 
dead  bark  over  the  invaded  portions  dries,  shrinks  and  cracks.  The 
larger  cracks  are  mostly  vertical,  the  smaller  horizontal  (fig.  2b). 
A  thin  layer  of  the  wood  immediately  under  the  invaded  bark  will 
usually  be  found  to  be  infiltrated  with  hardened  reddish-brown  gum 
which  protects  the  under  layers  from  rapid  drying  and  to  a  consid- 
erable extent  against  the  entrance  of  wood  rotting  fungi. 

On  old  sweet-orange  trees,  the  invaded  areas  are  usually  less  exten- 
sive and  more  restricted  laterally  than  on  the  common  lemon.  There 
is  usually  less  gum  than  on  the  lemon.  In  the  orange  there  is  a  greater 
tendency  for  the  invading  fungus  to  die  out  and  for  the  invaded 
area  to  become  self -limited  than  in  the  lemon.  Frequently  the  invaded 
areas  on  old  sweet-orange  trunks  extend  upward  from  the  soil  surface 
as  narrow  tongues  of  killed  bark.  On  younger  orange  trees  and 
frequently  on  older  ones  growing  vigorously  on  heavy  clay  soils  the 
disease  may  assume  much  the  same  characteristics  as  it  does  on  the 
common  lemon. 

INVESTIGATIONS  INTO  THE  NATUEE  AND  CAUSE  OF  THE  DISEASE 
Investigations  begun  in  1912  by  the  author19  have  led  to  the  dis- 
covery that  the  disease  is  infectious  and  that  the  causal  agent  is  a 
soil  inhabiting  "water  mold,"  Pythiacystis  citrophthora,  the  same 
fungus  which  was  previously  described  as  causing  brown  rot  of  lemon 

is  Ridgway,  Robert,  Color  standards  and  color  nomenclature,  43  pp.,  Wash- 
ington, D.  C,  1912. 

!9  Fawcett,  H.  S.,  Two  Fungi  as  causal  agents  in  Gummosis  of  lemon  trees 
in  California.     Calif.  State  Comm.  of  Hort.  Mo.  Bull.  2:001-617,  1913. 


10 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


fruits.20    The  relation  of  this  fungus  to  the  disease  was  discovered  as  a 
result  of  a  series  of  experiments  as  follows : 

1.  Inoculation  into  sound  trees  with  bits  of  diseased  tissue  trans- 
mitted the  disease  with  all  its  characteristic  symptoms  (fig.  2).  It 
was  found,  however,  that  only  the  diseased  tissue  from  the  marginal 
fringe  of  the  killed  bark  of  active  lesions  was  capable  of  transmitting 


Fig.  2. — Pythiacystis  of  brown  rot  gummosis  on  lemon  tree.  Produced  by 
inoculation  with  diseased  bark  tissue 

(a)  Tree  inoculated  February  27,  1912,  and  photographed  April  24,  1912. 
The  large  amount  of  exuded  gum  indicates  the  rapid  development  of  a  gummosis 
lesion.  A  majority  of  the  trees  in  this  same  orchard  had  the  same  over  growths 
at  the  union  of  stock  and  scion  as  observed  in  this  illustration. 

(&)  Same  view  as  (a)  on  May  24,  1913,  15  months  after  inoculation.  The 
gum  first  formed  has  been  dissolved  by  winter  rains,  and  the  dead  bark  has 
dried  and  shrunken.     Gum  was  exuding  (farther  around)  at  this  time. 


the  disease.  Tissue  from  places  away  from  this  margin  toward  the 
center  of  large  lesions  or  from  the  outer  gummous  zone  was  incapable 
of  inducing  gummosis. 

2.  Culture  tests  made  from  pieces  of  bark  from  various  places 
within   and  beyond   the  killed   portion   of   the   lesions   showed   that 


so  Smith,  R.  E.,  and  others,  The  Brown  rot  of  the  lemon,  Calif.  Agr.  Exp. 
Sta.  Bull.  190:1-72,  1907. 


BULL.  395]       BARK    DISEASES    OF    CITRUS    TREES    IN    CALIFORNIA  11 

Pythiacystis  citrophthora  was  alive  only  at  the  marginal  fringe  of 
what  was  subsequently  called  the  invaded  zone.  This  fungus  usually 
could  not  be  isolated  from  the  central  portion  of  the  invaded  zone 
and  it  was  never  found  in  the  outer  gummous  zone  itself  far  from 
the  margin  of  the  visibly  killed  area.  Isolations  of  this  fungus  were 
made  at  least  109  times  from  gummosis  trees  representing  30  different 
orchards  in  10  different  counties  of  California  extending  from  San 
Diego  on  the  south  to  Butte  county  on  the  north.  It  was  isolate  i 
also  from  one  locality  in  Arizona. 

3.  Inoculations  were  made  into  bark  of  sound  trees  with  pure 
cultures  of  this  fungus  and  the  disease  was  induced  with  all  its 
characteristic  symptoms  (figs.  3  and  4a).  Many  repetitions  gave 
the  same  results.  Inoculations  with  large  numbers  of  other  fungi 
and  bacteria  found  in  the  older  portions  of  the  invaded  zone  failed 
to  cause  the  disease.  The  reason  for  the  fact  that  only  diseased  tissue 
from  the  marginal  fringe  of  the  invaded  zone  was  capable  of  trans- 
mitting the  disease  was  now  revealed.  In  this  portion  only  does 
the  causal  organism  Pythiacystis  citrophthora  usually  remain  alive. 

4.  The  same  fungus  was  re-isolated  40  different  times  from  20  of 
the  cases  of  gummosis  produced  by  inoculation  and  again  found 
to  be  alive  only  at  the  outer  margin  of  the  invaded  zones,  just  as  in 
the  naturally  occurring  cases  previously  mentioned.  The  time  elaps- 
ing between  inoculation  and  re-isolation  was  from  1  to  12  months  in 
different  tests,  and  the  fungus  was  recovered  at  distances  of  20  inches 
or  more  from  the  original  point  of  inoculation.  One  strain  of  this 
fungus  isolated  from  a  diseased  tree  at  Whittier  was  inocLilated  into 
and  re-isolated  from  three  different  trees  in  succession  during  a 
period  of  3  years  from  1912  to  1915.  During  this  time  the  fungus 
lived  in  the  bark  of  the  three  trees,  for  periods  of  5,  11,  and  6  months1 
respectively  and  between  these  periods  in  cultures  for  2,  2,  and  10 
months  respectively.  Transfers  from  the  original  culture  were  kept 
alive  for  more  than  8  years  on  cornmeal  agar  medium  and  were  capa- 
ble of  producing  brown  rot  of  lemon  fruits  when  tested  in  1921. 

5.  Inoculations  were  made  also  with  bits  of  lemon  fruits  affected 
with  brown  rot  and  with  the  fungus  Pythiacystis  citrophthora  isolated 
from  diseased  fruits,  with  the  same  results,  as  those  obtained  by  the 
use  of  bits  of  diseased  bark  or  cultures  isolated  therefrom.  This 
experiment  served  to  show  that  the  fungus  previously  known  to  cause 
brown  rot  of  lemon  fruits  and  the  one  capable  of  inducing  this  type 
of  gummosis  were  identical.  The  detailed  experiments  on  which  these 
statements  are  based  will  be  found  elsewhere.21 


si  Gummosis  of  Citrus.  Jour,  of  Agr.  Res.  24:191-236,  1923. 


12 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


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BULL.  395]       BARK    DISEASES   OF    CITRUS   TREES   IN    CALIFORNIA 


13 


RESISTANCE  OF  DIFFERENT  SPECIES  AND  VARIETIES 

Among  the  Citrus  species  and  varieties  that  have  been  tested  the 
common  lemon  has  the  lowest  resistance  to  Pythiacystis  gummosis, 
and  the  sour  orange  the  highest.  The  sour  orange  usually  is  so 
resistant  to  Pythiacystis  attack  that  even  when  the  most  favorable 
conditions  are  given  by  inoculation  in  wounds,  there  is  only  a  slight 
gumming  with  rapid  healing  of  the  wounded  tissue  and  with  total 
failure  to  produce  a  diseased  lesion.  The  sour  orange  is  also  highly 
resistant  to  all  other  infectious  gum  diseases  of  importance.  Mere 
gum  formation,  however,  may  be  induced  by  suitable  stimuli  in  sour 
orange  as  well  as  other  species  and  varieties,  Of  the  forms  which 
have  been  most  used  for  stocks  in  California,  the  trifoliate  orange 
probably  stands  next  to  the  sour  orange  in  resistance  and  the  sweet 
orange  next  to  the  common  lemon  in  susceptibility,  with  the  grapefruit 
and  the  rough  lemon  standing  between  these  two.  Because  these  stocks 
are  grown  from  seed  there  is  a  possibility  of  variation  in  resistance 
within  each  variety  due  to  differences  between  strains  and  observations 
have  suggested  that  such  variation  actually  exists.  The  following 
observations  indicate  the  relative  resistance  of  some  of  the  common 
varieties. 

A  block  of  5000  sweet-orange  seedlings  about  two  and  one-half 
years  old,  growing  in  nursery  rows  on  medium  heavy  clay  loam  soil, 
had  been  planted  adjacent  to  a  block  of  15,000  sour-orange  seedlings 
of  the  same  age  and  receiving  the  same  care.  All  the  trees  had  been 
irrigated  rather  frequently  and  heavily.  On  October  21,  1914,  four 
representative  rows  of  sweet-orange  trees  showed  the  following  per- 
centage of  infection: 


Row 

Number  of  trees  in  row 

Number  of  affected  trees 

Percentage  affected 

1 

222 

52 

23 

2 

213 

73 

34 

3 

212 

63 

29 

4 

180 

53 

29 

Total 

827 

241 

Average  29 

On  some  trees  only  a  small  lesion  was  evident  with  much  gum  exud- 
ing, on  others  the  bark  was  killed  to  a  distance  of  6  to  12  inches  above 
the  soil,  with  an  abundance  of  gum,  and  still  other  trees  were  dead. 
Some  trees  showed  a  strong  tendency  to  form  ridges  of  callous  tissue 


14 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


along  the  edges  of  the  dead  strips  of  bark.     A  thorough  search  in 
the  block  of  sour-orange  trees  failed  to  reveal  a  single  affected  tree. 

Differences  in  resistance  are  indicated  further  by  an  estimate 
made  by  W.  M.  Mertz,  in  a  nursery  of  Citrus  seedlings  about  two  years 
old,  growing  at  the  Citrus  Experiment  Station.  The  following  is  the 
percentage  of  gum  disease  (probably  Pythiacystis  gummosis)  which 
was  recorded : 


Species 

Number  of 
trees 

1  F  Per  cent  with 
gummosis 

Citrus  Aurantium  (sour  orange) 

1000 
1000 
1000 
2000 

0.3 

Poncirus  trifoliata  (trifoliate  orange) 

1.0 

Citrus  maxima  (grapefruit) 

2.5 

Citrus  sinensis  (sweet  orange) 

10.00 

CONDITIONS  FACILITATING  INFECTION  AND  DEVELOPMENT  OF 

THE  DISEASE 

A  study  of  the  external  conditions  under  which  Pythiacystis  gum- 
mosis is  most  likely  to  become  severe,  taken  in  conjunction  with  experi- 
ments with  the  fungus,  indicates  that  the  important  factors  are :  ( 1 ) 
abundant  moisture  in  the  soil  in  contact  with  the  bark  over  a  suffi- 
ciently long  period,  (2)  favorable  soil  and  air  temperature,  and  (3) 
wounds  or  other  injuries  to  the  bark. 

Injuries,  however,  although  aids  to  infection,  are  not  necessary  if 
the  two  first-named  conditions  are  simultaneously  fulfilled.  Injuries, 
especially  below  the  soil  surface,  aside  from  inducing  infection  more 
readily  when  the  conditions  of  moisture*'  and  temperature  are  suitable, 
may  even  aid  in  bringing  about  infection  where  the  soil  moisture  is 
not  excessive  by  conveying  the  organism  into  the  moist  inner  bark 
tissue  where  penetration  can  start  without  the  addition  of  further 
moisture.  Wounds  or  injuries,  therefore,  while  not  absolutely  neces- 
sary for  the  entrance  of  the  parasite  when  sufficient  moisture  is  pres- 
ent are  probably  the  only  means  of  entrance  when  the  moisture 
content  of  the  soil  immediately  in  contact  with  the  bark  surfaces  is 
not  favorable  to  infection. 

Moisture. — The  severe  form  of  Pythiacystis  gummosis  is  most 
frequently  found  in  California  on  lemon  trees  where  they  are  budded 
low  on  sweet-orange  stocks  which  are  growing  on  heavy  soils  and  is 
especially  prevalent  in  the  coastal  sections.  In  the  light  of  what 
has  been  shown  this  is  readily  understood.  Heavy  clay  soils,  being 
much  more  retentive  of  moisture  than  lighter,  more  open  soils,  fur- 


Bull.  395 j     bark  diseases  of  citrus  trees  in  California  15 

nisli  the  means  by  which  water  films  may  remain  adjacent  to  the 
bark  for  periods  of  time  sufficient  to  permit  infection  by  the  parasite. 
The  great  prevalence  of  fogs,  and  the  comparatively  slight  fluctu- 
ations of  temperature  in  the  coastal  sections  also  decrease  the  rate 
of  evaporation  in  the  soil  adjacent  to  the  trees  that  has  become  exces- 
sively wet  during  irrigation  or  rain.  In  the  interior  valleys  even  the 
same  type  of  soil  usually  drys  out  more  quickly  after  irrigations  and 
rains.  If  the  drying  out  is  rapid  there  is  not  sufficient  time  for 
infection;  if  drying  is  slow,  however,  the  time  may  be  sufficient  for 
the  penetration  of  the  fungous  hyphae.  The  effect  of  a  low  bud  union 
is  mentioned  later.  The  lemon  bark  above  the  union  may  be  infected 
directly  when  the  fungus  has  communication  directly  with  it  through 
excessively  moist  soil  or  by  splashing  of  raindrops,  especially  when 
wounds  are  present.  High  budded  lemon  trees  are  less  liable  to  the 
disease.  If  budded  high  on  sweet-orange  stocks  they  are  more  likely 
to  escape,  because  this  stock  is  somewhat  more  resistant  than  lemon. 
It  is  nevertheless  susceptible  to  infection,  especially  under  conditions 
highly  favorable  to  the  disease.  If  budded  high  on  sour-orange  stock, 
however,  there  is  practically  no  danger  of  the  disease  even  under 
bad  water  conditions,  because  of  the  great  resistance  of  this  stock. 

Temperature. — Within  certain  limits,  temperature  appears  to  be 
another  important  factor,  especially  in  the  growth  of  the  fungus  and 
its  formation  of  spores  and  in  the  infection  of  the  host  as  a  result  of 
the  germination  of  these  spores.  As  has  been  previously  shown,22  a 
temperature  somewhere  near  25 °C.  (77°F.)  appears  to  be  about  the 
optimum  for  a  sustained  growth  of  the  fungous  hyphae  in  cultures, 
and  very  little  growth  takes  place  above  30°C.  (86°F.).  If  as  seems 
probable  similar  relations  hold  for  the  growth  of  the  fungus  in  the 
bark,  we  have  an  explanation  of  the  fact  that  the  disease  progresses 
slowly  during  hot  periods,  and  also  of  the  fact  that  its  progress  is 
sometimes  completely  checked,  the  fungus  being  killed  out  in  many 
instances.  This  occurs  especially  in  the  interior  valleys,  in  the  case 
of  parts  of  lesions  entirely  above  the  soil  surface.  Here  the  tempera- 
ture of  the  bark  frequently  reaches  many  degrees  above  the  death 
point  of  the  fungus.  Recent  experiments2'1  have  shown  that  a  temper- 
ature of  115 °F.  for  one  minute  is  sufficient  to  kill  the  spores  of  this 
fungus  on  infected  fruits  and  temperatures  much  lower  than  this  for 
greater  lengths  of  time  will  undoubtedly  have  the  same  effect. 


22  Fawcett,  H.   S.,   The  temperature  relations   of  gi'owth   in   certain   parasitic 
fungi,  Univ.  Calif.  Publ.  Agr.  Sei.,  4:183-232,  1921. 

23  Fawcett,  H.   S.,  Calif.  Citrograph,  7:233   and   254,   May,   1922. 


16  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Corroborating  this  view  is  the  observation  that  the  rapid  enlarge- 
ment of  invaded  areas  usually  continues  much  later  in  the  year  or 
may  proceed  throughout  the  whole  year  in  the  cooler  coastal  sections, 
while  the  activity  and  progress  of  the  disease  are  usually  limited 
to  the  winter,  spring  and  early  summer  in  the  hotter  interior  sections. 

Other  Conditions. — A  further  condition  which  contributes  to  the 
chance  of  infection  in  the  orchard  is  that  of  deep  planting,  and  the 
accumulation  of  soil  next  to  the  trunk  of  the  trees.  This  brings  a 
greater  surface  of  the  bark  in  contact  with  the  soil,  and  the  bud 
union,  if  low,  may  be  buried.  The  bark  over  the  union  between 
stock  and  scion  is  usually  especially  susceptible  to  infection,  probably 
because  of  frequent  irregularities  of  growth,  suckering,  etc.  The 
practice  of  loosening  the  soil  by  digging  about  the  base  of  the  tree, 
if  no  injuries  are  produced,  probably  helps  to  prevent  infection  by 
hastening  the  drying  out  of  the  soil.  This  practice  may,  on  the 
other  hand,  if  carelessly  performed,  result  in  injuries  to  the  bark 
and  aid  in  the  entrance  of  the  organism  or  actually  introduce  it  into 
the  inner  bark  tissues  on  bits  of  soil.  Any  agency  such  as  gophers, 
field  mice,  or  rabbits,  or  tools  used  in  orchard  operations  that  produce 
injuries  to  the  bark,  may  contribute  to  the  ease  of  infection  by  the 
parasite.  Infection,  however,  appears  to  take  place  frequently 
through  the  sound  uninjured  bark. 

The  following  is  a  concrete  example  of  a  set  of  conditions  that 
resulted  in  a  serious  outbreak  in  a  5-acre  6-year-old  lemon  orchard  at 
Whittier  in  1914,  on  medium  heavy  clay  soil. 

The  season  had  been  one  of  unusually  heavy  rainfall.  A  volunteer 
crop  of  barley  was  cut  while  green  in  March  and  thrown  around  the 
trees  in  contact  with  their  trunks.  This  operation  was  followed  by 
heavy  rains  and  cloudy  days.  During  the  following  May  90  trees  with 
brown-rot  gummosis  in  various  stages  were  discovered.  The  lesions 
appeared  to  have  started  at  various  places  below  and  above  the  surface 
of  the  soil.  Some  of  those  above  the  surface  had  started  at  places 
where  water  sprouts  had  been  broken  off,  but  many  appeared  to  have 
started  in  sound,  uninjured  bark.  The  barley  piled  around  the  trunks 
had  undoubtedly  aided  ia  retaining  films  of  water  for  a  sufficient  time 
to  permit  germination  of  the  spores  and  the  penetration  of  the  hyphae. 
All  the  diseased  trees  were  successfully  treated  under  my  direction 
by  the  methods  recommended  here. 


BULL.  395]       BARK   DISEASES   OF    CITRUS   TREES   IN    CALIFORNIA  17 


FUSARIUM  AS  AN  ACCESSORY  IN  THE  DEVELOPMENT  OF 
PYTHIACYSTIS  GUMMOSIS 

During  the  examination  of  a  large  number  of  naturally  occurring 
cases  of  Phythiacystis  gummosis  it  was  noticed  that  a  species  of 
Fusarium  frequently  accompanied  and  was  closely  associated  with  P. 
citrophthora  in  the  diseased  tissue.  The  question  arose  as  to  whether 
the  Fusarium  played  any  part  in  the  development  or  the  severity  of 
the  disease. 

Fusarium  has  been  mentioned  frequently  in  the  literature  as  hav- 
ing some  possible  relation  to  certain  types  of  gum  disease.  Briosi24 
and  McAlpine25  concluded  that  Fusarium  limoni  Briosi  played  an 
important  part  in  mal  di  gomma  in  Italy  and  in  Australia,  Earle  and 
Rogers26  though  not  able  to  produce  gummosis  by  inoculation  with 
Fusarium,  believed  that  under  certain  conditions  it  was  probably  a 
factor  in  a  certain  type  of  gum  disease  in  Cuba.  I  also  had  previously 
found  species  of  Fusarium  repeatedly  associated  with  mal  di  gomma 
or  footrot  in  Florida,  but  inoculations  with  them  had  given  negative 
results. 

Although  the  experiments  made  with  this  question  in  mind  have 
been  too  few  as  yet  to  allow  definite  conclusions  to  be  drawn,  certain 
results  have  indicated  that  the  severity  of  the  disease  is  slightly 
increased  by  adding  Fusarium  along  with  Pythiacystis  citrophthora 
at  the  time  of  inoculation.  The  characteristics  of  the  disease,  however, 
were  the  same  as  when  the  Pythiacystis  was  inserted  alone,  while 
inoculation  with  Fusarium  alone  failed  to  produce  this  type  of  gum- 
mosis. Fusarium  inoculations  caused  .only  a  slight  killing  of  a  narrow 
layer  of  tissue  along  the  cut  without  gumming  and  later  resulted  in  a 
cracking  of  outer  layers  of  bark  around  the  cut.  Otherwise  the  effect 
was  not  different  from  that  produced  in  the  uninoculated  cuts  used  as 
checks  on  the  same  trees. 


24  Briosi,  G.,  Intorno  al  mal  di  gomma  degli  agrumi  (Fusisporium  limoni, 
Briosi).  Atti  R.  Acad.  Lincei,  Roma,  ser.  3%  vol.  2;  Memoria  della  classe  di 
science  fisiche  etc.,  pp.  485-496,  1878. 

25  McAlphine,  D.,  Fungous  diseases  of  Citrus  trees  in  Australia  and  their 
treatment,  Melbourne,  p.  132,  1899. 

26  Earle,  F.  S.,  and  Rogers,  J.  M.,  "Citrus  diseases  at  San  Pedro  in  1915," 
in  San  Pedro,  Isle  of  Pines,  Citrus  Path.  Lab.,  Ann.  Report  1:36-38,  1915. 


18  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


METHODS  OF  CONTEOL 

Prevention. — The  causative  fungus  usually  infects  the  bark  at  or 
below  the  surface  of  the  soil.  The  roots  of  a  susceptible  variety  are 
more  resistant  than  the  bark  of  the  trunk.  As  pointed  out  above, 
some  of  the  principal  conditions  of  infection  are:  (1)  excessive  wet 
soil  in  contact  with  the  trunk,  as  a  result  of  improper  irrigation  and 
drainage,  or  long  periods  of  continuous  rains;  (2)  deep  planting,  or 
soil  piled  up  around  the  trunk,  especially  when  it  is  highly  retentive  of 
moisture;  (3)  injuries  to  the  bark  at  the  base  of  the  tree,  especially  in 
wet  weather. 

The  methods  which  have  been  found  effective  and  are  now  in  com- 
mon use  in  preventing  infection  brought  about  by  these  conditions, 
are  those  which  follow : 

The  soil  is  pulled  back  from  the  base  of  the  tree  to  expose  the  top 
of  the  first  main  roots  and  is  left  in  a  circular  riclge  to  exclude  irriga- 
tion waters  from  the  depression  next  to  the  trunk.  If  the  depression 
occasionally  becomes  filled  up  with  water,  the  resulting  condition  is 
usually  not  so  serious  as  when  the  soil  is  against  the  bark,  because 
the  water  after  a  rain  soon  percolates  into  the  soil.  Under  usual 
weather  conditions  in  California,  with  most  Citrus  soils,  the  length  of 
time  that  water  would  stand  in  such  a  depression  is  not  sufficient  to 
produce  infection. 

This  pulling  away  of  the  soil  is  most  important  on  the  heavier  clay 
soils.  On  looser  sandy  soils  it  is  not  so  necessary. 

As  an  additional  means  of  prevention  the  bark  at  the  base  of  the 
trunk  is  painted  with  Bordeaux  paste  or  other  non-injurious  fungicide 
after  pulling  the  soil  away.  (For  results  of  extensive  experiments  in 
prevention  see  Bull.  360,  Calif.  Exp.  Sta.  1923.) 

A  means  of  prevention  especially  on  very  heavy  soils,  is  to  use 
trees  budded  high,  1  to  2  feet  or  more,  on  resistant  stocks,  such  as 
a  good  type  of  the  sour-orange.  The  sour-orange  stock  is  resistant, 
not  only  to  Pythiacystis  gummosis  but  also  to  psorosis  and  other  gum 
diseases. 

Treatment. — Even  when  careful  attention  is  given  to  prevention,  a 
few  cases  of  Pythiacystis  gummosis  will  often  occur  from  time  to 
time,  especially  with  trees  on  susceptible  stocks  planted  in  heavy  soils. 
The  following  method  of  treatment  has  been  adopted  as  the  result  of 
much  experimental  work  in  which  the  cooperation  of  growers  has 
been  of  great  assistance.27     The  extent  to  which  the  bark  is  killed 


27  Fawcett,  H.  S.,  Calif.,  State  Comm.  of  Hort.  Mo.  Bull.  2:601-617,  1913,  and 
Prizer,  J.  A.,  Calif.  State  Comm.  of  Hort.  Mo.  Bull.  4:7-19,  1915. 


B    LL.  395]       BARK    DISEASES    OF    CITRUS    TREES    IN    CALIFORNIA  19 

through  to  the  wood  is  first  ascertained  by  scraping  slightly.  Then 
the  brownish  killed  bark  (invaded  zone)  is  dissected  ont  with  a  heavy 
knife,  cutting  through  to  the  wood  about  %  inch  beyond  the  invaded 
zone  on  the  sides  and  1  to  2  inches  beyond  at  the  top  and  bottom 
in  a  manner  similar  to  that  shown  in  figure  4&.  The  cuts  on  the  two 
sides  are  usually  brought  together  above  and  below,  making  an  acute 
angle.  No  attempt  is  made  to  cut  beyond  the  outer  gummous  zone, 
since  it  has  been  shown  that  this  zone  does  not  contain  the  invading 
parasite.  The  live  bark  over  this  zone  of  gummous  influence,  not  yet 
invaded  by  Pythiacystis  citrophthora,  will  usually  recover  rapidly  as 


- 

r 

■                                                   C 

* -'    "     *: .-'  ^H                         i  '.iteaSSfeftei 

Fig.  4. — Method  of  cutting  away  diseased  bark  in  treatment  of  a  severe 
case  of  Pythiacystis  gummosis. 

(a)  Result  of  inoculation  with  a  bit  of  diseased  bark  on  November  16,  1912. 
Photographed  September  3,  1913.     Invaded  area  3X8.5  inches. 

(&)  Bark  cut  away  September  3,  1913.  Ready  for  painting  with  Bordeaux 
paste. 

(c)  Same  tree  on  March  17,  1916  (Sy2  years  later)  showing  growth  at  edges 
of  wound  and  asphalt  paint  on  exposed  wood. 

soon  as  the  invaded  portion  has  been  removed  and  the  further  progress 
of  the  fungus  has  been  stopped.  These  cut-out  places  and  the  entire 
trunk  are  then  painted  with  Bordeaux  paste  (1  lb.  copper  sulphate 
and  2  lbs.  rock  lime  to  l1/^  gallons  of  water)  or  some  other  suitable 
fungicide  (see  formulae  under  "Psorosis").  The  fungicides  other 
than  Bordeaux  paste  which  have  been  most  generally  used  in  California 
are  coal  tar  products  from  which  the  lighter  oils  have  been  distilled 
such  as  Arrow  Carbolineum  and  Avenarius  Carbolineum. 


20  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Other  safe  fungicides  of  known  composition  are:  mercuric  chloride 
1  gram  to  1000  cc  of  25  per  cent  denatured  alcohol  (%  ounce  to  one 
gallon)  or  mercuric  cyanide  of  the  same  strength;  emulsified  cresol 
(liquor  cresolis  compositus  USP)  10  per  cent  in  water;  or  potassium 
permanganate  crystals  1  per  cent  in  water  (one  ounce  in  3  quarts  of 
water).  The  mercuric  chloride  and  mercuric  cyanide  may  be  obtained 
as  tablets  or  in  crystalline  form,  the  potassium  permanganate  and 
copper  sulphate  in  crystalline  form,  the  liquor  cresolis  compositus 
and  carbolineums  in  liquid  form.  These  are  sold  by  drug  stores,  drug 
companies  and  spray  dealers. 

Before  applying  to  the  bark  carbolineum  or  any  other  product  the 
exact  composition  or  effect  of  which  is  unknown,  it  should  be  tested 
on  some  leaves  and  twigs  several  days  before  it  is  to  be  used  on  the 
bark.  If  in  two  or  three  days  it  does  not  seriously  injure  the  mature 
leaves  or  twigs  it  will  probably  be  safe  to  apply  to  the  bark.  If, 
however,  the  leaves  are  severely  burned,  the  product  is  usually  not 
safe  to  use  on  the  bark. 

The  elimination  of  the  tissue  invaded  by  the  causative  organism  is 
the  most  important  feature  of  the  treatment,  the  purpose  of  the  fungi- 
cide being  to  prevent  reinfection,  or  to  kill  the  organism  in  small  bits 
of  tissue  left  behind. 

After  treatment  the  gum  usually  continues  to  exude  for  some  time 
at  the  edges  of  the  cut-out  areas.  This  is  left  undisturbed  except  to 
examine  the  edges  of  the  cuts  to  ascertain  whether  any  bark  is  being 
killed  by  further  invasion  of  the  fungus.  The  gum  appears  to  act  as  a 
protective  covering  under  which  callous  tissue  is  readily  formed  along 
the  edges  of  the  cuts.  After  these  cut  edges  begin  to  heal,  the  exposed 
wood  is  painted  with  asphalt  paint  or  other  good  covering  (fig.  4c). 

In  cases  where  a  large  part  of  bark  on  the  trunk  has  been  killed 
by  the  rapid  invasion  of  Pythiacystis  citrophthora,  so  that  the  trunk 
is  girdled  or  nearly  so,  bridge  grafting28  and  inarching29'  30>  81»  32' 
may  often  be  resorted  to  in  order  to  save  the  tree  (fig.  5). 

The  advisability  of  this  practice,  in  any  particular  instance,  will 
depend  upon  the  age  and  condition  of  the  tree  and  other  factors. 
A  modification  of  the  usual  method  of  bridge  grafting  has  been  suc- 


28  Paddock,  E.  H.;  Bridge  grafting  on  Citrus,  Calif.  State  Comm.  Hort.  Mo. 
Bull.  81:72-73,  1919. 

soRundle,  H.  E.,  Inarching  Citrus  trees.     Calif.  Cultivator,  44:540-541,  1916. 

so  Hume,  H.  H.,  Citrus  fruits  and  their  culture,  ed.  4,  196  pp.,  New  York, 
1911. 

31  Fawcett,  H.  S.,  Experiments  in  bridge  grafting  and  inarching  in  connec- 
tion with  Gummosis  of  Citrus.     Calif.  Citrograph,  8:68  and  95,  1923. 

32  Hodgson,  R.  W.,  Saving  the  gophered  Citrus  tree,  Calif.  Agr.  Exp.  Sta. 
Cir.  2:73:1-20,  1924. 


BULL.  395]       BARK    DISEASES   OF    CITRUS   TREES   IN    CALIFORNIA 


21 


cessful  with  Citrus.  Instead  of  a  shoot  or  branch  being  grafted  into 
the  bark  both  above  and  below  the  injured  portion,  a  sprout  at  or 
below  the  lower  edge  of  the  wound  is  allowed  to  grow.  When  of 
sufficient  size  the  top  part  is  cut  off,  bent  over,  and  grafted  into  the 
bark  above  the  diseased  area  (fig.  5&). 


Fig.  5. —  (a)  Young  lemon  tree  into  which  a  sour-orange  seedling  %  inch  in 
diameter  had  been  inarched  over  a  gummosis  lesion  about  2%  years  before.  The 
inarched  seedling  had  increased  to  %  inch  in  diameter  and  had  caused  a  ridge 
of  growth  above  as  shown  alongside  the  white  chalk  mark. 

(&)  Old  lemon  tree  showing  gummosis  lesion  bridged  by  inarching  sprouts 
from  the  base.     About  5  years  after  inarching. 

Another  similar  method  of  saving  or  aiding  an  injured  tree  is 
inarching,  in  which  one  or  more  small  seedling  trees  are  planted  close 
to  the  trunk  and  the  top  grafted  into  the  bark  above  the  injury. 
Where  quick  relief  is  necessary  and  suitable  sprouts  cannot  be  stimu- 
lated, this  method  has  proved  beneficial  (fig.  5a).  (For  details  as 
to  extensive  experiments  in  treatment  see  Bull.  360,  Calif.  Agr.  Exp. 
Sta.) 

An  example  of  what  can  be  done  in  controlling  Pythiacystis  gum- 
mosis when  it  is  treated  promptly  and  with  care,  is  furnished  by  a 


22 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


five-acre  lemon  orchard  at  Whittier  on  clay  loam  soil,  previously 
referred  to,  where  conditions  facilitating  infection  obtained.  About 
20  per  cent  of  the  six-year-old  trees  became  infected  after  a  period  of 
hea\y  rains  which  followed  the  application  of  green,  freshly  cut  barley 
about  the  trunks  in  March,  1914. 

The  lesions  noticed  in  May  varied  in  size  from  those  just  started 
to  lesions  extending  one-half  to  two-thirds  around  the  circumference 


* 

f 

«^B 

(  i 

flffi 

Fig.  6. — Showing  a  method  formerly  used  in  treating  brown-rot  gummosis 
by  cutting  vertical  slits  in  the  bark,  one  on  each  side  of  the  diseased  lesion 
and  several  down  through  the  invaded  area.  This  treatment  appeared  to  be  of 
some  value  for  mild  cases,  but  was  useless  for  more  severe  outbreaks. 


Bull.  395]       BARK    DISEASES   OF    CITRUS   TREES   IN    CALIFORNIA  23 

of  the  bark  on  the  trunk.  On  May  25,  1914,  the  trees  were  treated  by 
dissecting  out  all  the  brown  killed  bark  (invaded  zone)  and  cutting 
about  %  inch  beyond  this  invaded  zone  on  the  sides  and  from  1  to  2 
inches  beyond  at  the  top  and  bottom,  as  is  shown  in  fig.  46.  The  cuts 
on  the  two  sides  were  usually  brought  together  above  and  below, 
making  an  acute  angle. 

No  attempt  was  made  to  cut  beyond  the  outer  gummosis  zone  of 
gummous  influence,  so  that  in  most  cases  on  larger  lesions  this  zone 
probably  extended  into  the  remaining  bark  a  considerable  distance 
beyond  the  cuts.  As  has  been  pointed  out  in  the  description  of  the 
disease  and  elsewhere,  the  margin  of  the  zone  actually  invaded  by  the 
causal  fungus  usually  lags  far  behind  the  zone  in  which  gum  forma- 
tion occurs.  These  cut-out  places  and  the  entire  trunks  were  painted 
with  Bordeaux  paste  (1  lb.  copper  sulfate  and  2  lbs,  rock  lime  to  1% 
gallons  water).  In  order  to  prevent  further  infection  the  soil  was 
pulled  back  from  the  trunk  as  far  as  the  top  of  the  first  main  roots, 

After  treatment,  the  gum  continued  to  exude  in  considerable 
quantities  at  the  edges  of  the  cut-out  areas,  This  gum  was  left  undis- 
turbed except  for  examining  the  edges  of  the  cuts  to  ascertain  whether 
the  bark  was  being  killed  further.  The  gum  appeared  to  act  as  a 
protective  covering  under  which  callous  tissue  was  soon  seen  to  be 
forming  rapidly  along  the  edges  of  the  cuts. 

On  June  22,  1914,  about  one  month  after  treatment,  an  inspection 
showed  only  five  trees  with  new  areas  of  killed  bark,  all  of  which 
areas  were  on  treated  trees.  On  August  3,  seven  additional  areas  of 
killed  bark  were  found,  and  on  September  15,  eight  additional  areas, 
all  except  two  of  which  were  on  treated  trees. 

On  March  29,  1915,  a  rapid  healing  was  noted  at  all  the  cuts  and 
no  effect  on  the  foliage  could  be  detected.  Three  new  lesions,  all 
originating  below  the  surface  of  the  soil,  were  found. 

On  July  30,  1915,  three  trees  which  had  lost  bark  from  areas  extend- 
ing over  two-thirds  to  three-fourths  their  trunk  circumferences,  were 
showing  a  slight  yellowing  of  the  foliage,  but  only  on  branches  directly 
above  the  dissected  areas.  By  March  6,  1916,  however,  only  one  tree 
showed  any  appreciable  effect  of  the  removal  of  bark  and  this  was  one 
from  which  about  120  square  inches  had  been  removed.  The  trunks 
of  all  the  trees  had  been  painted  again  with  Bordeaux  paste  in  the 
fall  of  1915,  and  no  new  cases  were  found  during  the  spring  and 
summer  of  1916. 

The  rapidity  with  which  cut-out  places  of  various  sizes  healed  is 
indicated  by  table  2.  These  figures  were  obtained  by  measuring  the 
areas  in  two  directions  and  estimating  the  number  of  square  inches 


24 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


in  each.  While  the  figures  are  only  approximate,  they  give  a  rough 
idea  at  least  of  the  rate  at  which  areas  of  different  sizes  on  lemon  trees 
of  this  age  may  be  expected  to  close  up  with  new  bark  tissue. 


Table  2 


Number  of  areas 

Average  size  of  original 
areas,  May,  1914 

Average  size  of  same 
areas,  June,  1916 

Average  size  of  same 
areas,  April,  1921 

sq.  in. 

sq.  in. 

sq.  in. 

5 

.5 

0.0 

0.0 

3 

1.0 

.08 

0.0 

8 

2.0 

.97 

0.0 

4 

3.6 

1.2 

0.0 

10 

4.0 

2.7 

0.6 

2 

5,0 

3.5 

0.5 

5 

6.0 

3.7 

0.0 

4 

7.0 

2.5 

0.9 

7 

8.0 

4.0 

0.6 

5 

10.0 

4.2 

1.0 

2 

11.0 

8.0 

2.0 

5 

12.0 

9.8 

3.2 

3 

16.0 

11.3 

5.0 

5 

22.0 

17.0 

5.6 

4 

35.0 

27.0 

7.5 

3 

40.0 

22.0 

15.0 

7 

50.0 

42.5 

25.0 

4 

75.5 

55.0 

23.7 

5 

105.6 

69.0 

39.6 

1 

130.0 

80.0 

60.0 

It  will  be  noted  that  all  wounds  less  than  3  square  inches  in  size 
when  treated  were  closed,  or  insignificant,  at  the  end  of  2  years. 
Nearly  all  wounds  less  than  10  square  inches  in  size  when  treated 
were  closed  in  7  years.  Areas  larger  than  10  square  inches  were 
reduced  to  about  one-half  to  two-thirds  of  their  original  size  in  2 
years  and  to  about  one-third  to  one-fourth  their  original  size  in  7 
years. 

MAL  DI  GOMMA,  OK  FOOT  ROT 

Mai  di  gomma,  due  to  Phytophthora  terrestris  Sherbakoff,J  is  a 
gum  disease  with  close  relationships  to  Pythiacystis  gummosis.  Cer- 
tain phases  of  the  Pythiacystis  gummosis  occurring  on,  or  near,  the 
main  roots  of  sweet-orange  trees,  are  quite  similar  to  those  of  mal  di 
gomma  or  foot  rot.  For  this  reason,  certain  foot  rot-like  forms,  due 
to   Pythiacystis    citrophthora   in    California,    have    previously    been 

t  Phytophthora  terrestris  is  by  some  authors  considered  to  be  the  same  as 
P.  parasitica. 


BULL.  395]       BARK   DISEASES   OF    CITRUS   TREES   IN    CALIFORNIA  25 

referred  to  as  mal  di  gomma,33,  34  Since  the  term  mal  di  gomma  was 
used  in  Florida  to  designate  a  common  Florida  gum  disease,  which 
is  now  known  to  be  induced  by  Phytophthora  terrestris,  it  is  proposed 
to  restrict  its  use  (in  this  country  at  least)  to  the  disease  due  to  this 
fungus. 

SYMPTOMS 

This  type  of  gum  disease  affects,  for  the  most  part,  the  bark  on 
the  lowest  portion  of  the  trunk  and  the  upper  portion  of  the  highest 
main  roots,  mostly  below  the  surface  of  the  soil.  Gum  usually  forms 
on  the  trunk  of  the  tree  above  the  soil.  The  inner  bark  and  finally 
the  wood  underneath  frequently  develop  a  fetid  odor.*  The  bark 
dies  and  breaks  away  in  patches,  leaving  bare,  dead  areas,  which 
spread  in  all  directions,  but  mostly  downward,  on  the  main  crown 
roots  and  laterally  around  the  trunk  (fig.  8).  Trees  thus  affected  bear 
heavy  crops  of  fruit  temporarily  and  the  leaves  become  yellow. 

HISTORY 

This  gum  disease  was  first  known  as  foot  rot  and  attracted  atten- 
tion in  Florida  about  the  same  time  that  Pythiacystis  gummosis  was 
noticed  in  California.  Curtiss,35  in  1888,  reported  that  it  appeared  in 
1876,  although  few  people,  he  says,  remember  having  observed  it 
before  1880.  That  the  disease  was  not  important  in  Florida  before 
that  time  is  indicated  by  the  fact  that  Bishop,36  writing  on  Citrus 
culture  in  Florida  in  1875,  discusses  a  number  of  other  diseases  but 
does  not  mention  this  one. 

In  a  book  published  in  1881,  Moore,37  who  made  extensive  observa- 
tion on  Citrus  culture  in  Florida,  speaks  of  this  disease  under  "foot 
rot"  as  having  appeared  in  "late  years"  in  Florida.  In  1896  Swingle 
and  Webber38  stated  that  the  disease  was  widely  distributed  in  Florida 
and  seemed  to  be  gradually  spreading. 

33  Smith,  R.  E.,  and  Butler,  O.,  Gum  disease  of  Citrus  trees  in  California. 
Calif.  Agr.  Exp.  Sta.  Bull.  200:235-272,  1908. 

34  Fawcett,  H.  S.,  The  known  distribution  of  Pythiacystis  citrophthora,  and  its 
probable  relation  to  mal  di  gomma.    Phytopathology,  5:66-67,  1915. 

*  The  rotting  of  the  wood,  as  well  as  the  bark,  and  the  accompanying  fetid 
odor  are  believed  to  be  due  mainly  to  secondary  organisms  setting  up  fermenta- 
tion and  decay  below  the  surface  of  the  soil  after  the  killing  of  the  bark  by 
the  primary  organism.  While  gum  may  be  formed  below  as  well  as  above  the 
surface  of  the  soil,  it  is  dissolved  readily  by  moisture  and  is  usually  less  con- 
spicuous below  the  soil  surface.  This  disease,  under  California  conditions, 
cannot  be  distinguished  from  certain  phases  of  brown  rot  gummosis  except  by 
means  of  culture  tests  for  isolating  the  causal  organisms. 

35  Curtiss,  Sore  shin  or  gum  disease.     Fla.  Agr.  Exp.  Sta.  Bull.  2:27-42,  1888. 

36  Bishop,  P.  P.,  Proc.  Am.  Porno.  Soc,  p.  48,  1875. 

37  Moore,  T.  W.  Treaties  and  handbook  of  orange  culture,  New  York,  1881. 

38  Swingle,  W.  T.,  and  Webber,  H.  J.,  The  principal  diseases  of  Citrus  fruits 
in  Florida,    U.  S.  D.  A.,  Div.  of  Veg.  Phys.  and  Path.  Bull.  8:1-40,  1896. 


26  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


INVESTIGATIONS 

Only  once  has  the  causal  fungus  been  isolated  in  California.  This 
was  from  an  orange  tree  at  Lindsay  in  1912.  It  was  considered  by 
the  writer  at  that  time  to  be  only  a  peculiar  strain  of  the  brown  rot 
fungus,  Pythiacystis  citrophthora,  but  was  later  identified  by  Sherba- 
koff  as  the  same  species  which  he  had  previously  described.39  This 
species,  or  a  closely  allied  one,  appears  to  be  widely  distributed,  occur- 
ring in  a  number  of  countries.  It  was  found  on  Citrus40'  41  in  Florida, 
Cuba  and  Argentine ;  on  coconuts,  tobacco  and  pineapples  in 
Jamaica  ;42  on  tomatoes  in  Florida39  and  castor  oil  plants  and  Vinca 
in  India.43 

Many  comparative  inoculations,  with  Pythiacystis-  citropththora, 
the  cause  of  brown  rot  gummosis,  and  Phytophthora  terrestris,  the 
cause  of  mal  di  gomma,  were  made  under  various  conditions  and  the 
same  type  of  lesion  was  produced  by  each  fungus.  There  was  an  indi- 
cation in  some  of  the  experiments  that  sweet-orange  bark  was  more 
susceptible  than  lemon  to  Phytophthora  terrestris  while  the  reverse 
was  generally  true  of  the  fungus  of  brown-rot  gummosis.  According 
to  Hume,44  in  Florida  the  sweet  orange  is  more  susceptible  to  mal  di 
gomma  than  the  common  lemon,  and  sour-orange  is  very  resistant. 

The  relation  of  temperature  to  growth  is  somewhat  different  for 
the  two  fungi.45  While  the  optimum  temperature  for  sustained  growth 
over  a  period  of  several  days  in  the  laboratory  for  Pythiacystis  citroph- 
thora is  about  77°  F.  (25°  C),  that  for  Phytophthora  terrestris  is 
about  86°F.  (30°C).  It  is  of  interest  in  this  connection  that  the 
rainy  season  in  California  occurs  during  winter  and  spring,  coinci- 
dent with  moderate  temperatures,  while  the  moist  season  in  Florida 
occurs  during  the  summer,  coincident  with  much  higher  temperature. 


39  Sherbakoff,  C.  D.,  Buckeye  rot  of  tomato  fruit.  Phytopathology,  7:119-129, 
1917. 

40  Fawcett,  H.  S.,  Pythiacystis  and  Phytophthora.  Phytopathology, 
10:397-399,  1920. 

4i  Stevens,  H.  E.,  Florida  Citrus  diseases.  Fla.  Agr.  Exp.  Sta.  Bull. 
150:1-110,  1918. 

42  Ashby,  S.  F.,  Leaf-stalk  rot  caused  by  Phytophthora  parasitica.  In  West 
Indian  Bull.  18:70-73,  1920. 

43  Dastur,  J.  F.,  Phytophthora  parasitica  new  species;  a  new  disease  of  the 
castor  oil  plant.  Mem.  Dept.  Agr.  India,  Bot.  Ser.,  5:177-231,  1913;  idem., 
Phytophthora  on  Vinca  rosea.    Mem.  Dept.  Agr.  India,  Bot.  Ser.,  8:233-242,  1916. 

44  Hume,  II.  IT.,  Some  Citrus  troubles.  Fla.  Agr.  Exp.  Sta.  Bull,  53:145-173, 
1900. 

45  Fawcett,  H.  S.,  The  temperature  relations  of  growth  in  certain  parasitic 
fungi.     Univ.  Calif.  Publ.  Agr.  Sci.,  4:183-232,  1921. 


BULL.  395]       BARK    DISEASES   OF    CITRUS   TREES   IN    CALIFORNIA 


27 


Fig.  7. — Mai  di  gomma  or  footrot.  (a)  On  40-year-old  seedling  orange  trees. 
White  lines  indicate  boundaries  of  killed  bark,  (b)  Earth  dug  away  and  affected 
bark  and  roots  being  removed  before  applying  Bordeaux  paste. 


28  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


CONTEOL 

The  means  of  prevention  and  treatment  of  mal  di  gomma,  are 
essentially  the  same  as  those  for  brown-rot  gummosis,  a  certain  form 
of  which  it  so  closely  resembles  that  only  laboratory  examination  can 
distinguish  the  two  diseases.  Care  regarding  excessive  moisture  in 
contact  with  the  bark  of  the  crown  roots  and  the  base  of  the  trunk, 
care  in  keeping  the  soil  away  from  the  base  of  the  tree  and  in  avoiding 
injuries,  the  use  of  sour-orange  stocks  for  new  plantings,  the  use  of 
fungicidal  washes  on  the  bark,  are  all  useful  in  prevention.  For 
detailed  suggestions  regarding  prevention  and  treatment,  see  under 
brown-rot*  gummosis. 


DRY  ROOT  ROT 

A  disease  of  the  roots  known  as  dry  root  rot  is  of  considerable 
importance  in  California.  Root  diseases  of  the  same  general  nature 
have  also  been  described  in  other  countries.  This  root  rot  has  been 
designated  by  the  term  "dry"  to  distinguish  it  from  the  wet  or  disin- 
tegrating types  of  rot  like  that  of  Armillaria  and  certain  phases  of  mal 
di  gomma,  etc.  It  is  only  in  its  later  stages,  however,  that  it  is  really 
a  dry  root  rot. 

Symptoms. — Decay  of  the  bark  and  wood  takes  place  on  roots 
at  and  below  the  crown  and  usually  below  the  surface  of  the  soil. 
In  the  active  stages  of  the  decay,  especially  when  it  is  starting,  the 
bark  is  moist  and  if  the  soil  is  wet  there  may  be  a  semi-mushy  condi- 
tion. This  is  not  so  pronounced  or  soft  as  in  certain  other  root  rots. 
Usually  no  gum  is  apparent,  but  occasionally  a  small  amount  is 
visible  at  the  advancing  edge  of  the  active  lesions.  The  disease  is  often 
associated  with  mal  di  gomma  or  Pythiacystis  gummosis  in  which 
case  considerable  gum  may  be  present. 

Large  or  small  lesions  may  occur  on  roots  at  little  distance  from 
the  crown  or  the  entire  crown  and  base  of  main  roots  may  be  involved 
in  varying  degrees.  It  may  start  on  the  side  of  a  tap  root  or  on  large 
lateral  roots  and  later  spread  to  the  crown.  The  bark  is  at  first  soft, 
soggy  and  abnormally  dark.  When  further  decay  sets  in,  it  becomes 
dryer  and  crumbles  away.  Hard  dead  wood  is  found  underneath. 
There  is  a  tendency  in  some  cases  for  the  lesions  to  enlarge  for  a 
time  and  then  become  self -limited  by  a  production  of  callus.  Smaller 
lesions  may  be  partially  covered  with  new  bark  as  time  goes  on. 
Unlike   pure   Pythiacystis   gummosis  or   mal   di   gomma   the   wood 


BULL.  395]       BARK    DISEASES   OF    CITRUS   TREES   IN    CALIFORNIA  29 

usually  becomes  affected  for  some  distance  inward.  The  wood  becomes 
light  brown,  and  remains  firm  as  the  dry  root  rot  proceeds.  This  dry 
decay  in  the  wood  frequently  involves  a  greater  part  of  the  woody 
cylinder  of  the  root  and  consequently  has  a  severe  effect  upon  the 
tree.  The  symptoms  above  the  ground  are  usually  manifested  in  an 
open  thin  appearance  of  the  tree,  yellowing  and  dropping  of  foliage 
and  frequently  the  setting  of  an  unusually  large  crop  of  fruit.  The 
effects  are  much  the  same  as  if  the  trees  were  gophered  or  had  severe 
mal  di  gomma  or  Pythiacystis  gummosis.  The  leaves  may  wilt  sud- 
denly and  the  entire  foliage  and  twigs  dry  up  in  a  few  days  or  the 
tree  may  have  a  lingering,  more  gradual  death  in  which  it  first  becomes 
slowly  defoliated. 

Contributing  conditions  and  probable  cause. — The  investigations 
of  Barrett46  in  California  have  shown  that  fungous  species  of  Fusa- 
rium  are  nearly  constantly  associated  with  dry  root  rot  lesions.  He 
states :  ' '  The  fact  that  different  species  of  Fusarium  have  been  isolated 
from  different  cases  of  the  disease  occurring  in  different  sections,  would 
indicate  that  closely  related  types  of  dry  root  rot  may  be  induced  by 
several  species  of  the  genus  Fusarium."  I  also  have  found  Fusaria 
commonly  in  the  dry  root  rot  lesions.  Inoculations  by  Barrett  from 
pure  cultures  of  Fusaria  as  well  as  from  diseased  tissue,  have  so 
far  failed  to  reproduce  the  disease  in  its  severe  form.  I  have  made 
similar  inoculations  with  the  same  results.  Only  small  lesions  have 
resulted. 

Some  of  the  conditions  thought  to  be  important  are  excessive 
moisture,  poor  aeration,  heavy  soils  and  possibly  injuries  to  the 
roots  such  as  those  due  to  larvae  of  Fuller's  rose  beetle,  rodents,  heavy 
fumigation  with  cyanide,  etc.  Excessive  moisture  at  the  base  of  the  tree 
is  probably  one  of  the  most  important.  The  lack  of  aeration  would 
naturally  accompany  or  follow  excessive  moisture.  Heavy  soil  while 
possibly  a  factor  is  not  necessary  to  its  occurrence.  Some  very  severe 
cases  have  been  found  in  sandy  soil. 

Control. — No  thoroughly  satisfactory  method  of  control  appears 
feasible  for  advanced  stages  of  the  dry  root  rot  since  the  wood  as  well 
as  the  bark  is  affected.  The  method  of  cutting  away  only  the  diseased 
bark  as  in  mal  di  gomma  and  Pythiacystis  gummosis  is  not  sufficient. 
If  only  certain  roots  are  affected  and  the  tap  root  and  crown  are 
still  in  good  condition,  these  diseased  roots  should  be  cut  out  and 
destroyed.  It  may  be  advisable  if  a  lesion  is  not  too  far  advanced  at 
the  crown  of  a  large  tree  to  carefully  cut  some  distance  beyond  the 


46  Barrett,  J.   T.,  Annual  Rept.  of  Calif.    Citrus    Institute,    1919-20,    p.    157, 
1920. 


30  UNIVERSITY    OF    CALIFORNIA- — EXPERIMENT    STATION 

visible  margin  of  the  dry  decay  in  the  wood  as  well  as  the  bark.  The 
wound  should  then  be  disinfected  with  mercuric  cyanide  or  mercuric 
chloride  (1  to  500)  or  Arrow  carbolineum  and  a  little  later  covered 
with  thick  asphalt  paint  or  tar.  Wounds  made  in  cutting  off 
large  roots  may  be  treated  in  the  same  manner.  Preventive  measures 
consist  in  keeping  away  excessive  water,  especially  from  the  crown, 
providing  good  aeration  and  drainage,  and  avoiding  injuries  to 
the  roots  as  far  as  possible.  These  precautions  will  also  help  in  pre- 
venting Pythiacystis  gummosis,  mal  cli  gomma  and  other  troubles  that 
depend  to  a  considerable  extent  on  the  same  contributing  conditions. 
Under  some  conditions  of  very  heavy  soil  a  preventive  Bordeaux  wash 
applied  to  the  base  of  the  large  roots  and  crown  may  also  be  useful. 


ARMILLARIA  ROOT  ROT 

This  disease,  known  also  as  oak  root  fungus  and  fungus  root  rot, 
is  troublesome  in  certain  sections  of  California,  especially  in  localities 
where  affected  oaks  have  been  cut  down  and  Citrus  trees  planted. 
The  disease  is  due  to  a  toadstool  fungus,  Armillaria  mellea. 

More  or  less  gumming  often  accompanies  this  disease.  It  is,  there- 
fore, included  under  the  gum  diseases.  The  gum  oozes  out  at  or  just 
above  the  base  of  the  trunk,  and  might  be  confused  with  that  due  to 
some  forms  of  gummosis.  This  disease  is  characterized  by  a  decay 
of  the  roots,  and  of  the  bark  and  wood  at  the  base  of  the  tree,  and 
by  the  formation  of  black  strands  on  the  roots  and  of  fan-shaped, 
felted,  white  growths  which  crowd  into  the  live  bark.  It  is  accom- 
panied by  a  mushroom  odor  and  by  the  development  of  toadstools  of 
Armillaria  mellea  near  the  base  of  the  tree  in  the  fall  and  winter. 

The  fungus  slowly  spreads  from  the  roots  of  an  affected  tree  to 
the  roots  of  the  surrounding  healthy  trees,  often  killing  a  tree  in  each 
direction  in  from  one  to  four  years.  W.  T.  Home,47  who  has  inves- 
tigated this  disease  in  California,  states  that  trees  not  girdled  by  the 
fungus  may  be  kept  alive  for  many  years  by  digging  around  the 
tree,  removing  all  diseased  roots  and  decayed  patches  of  bark  and 
disinfecting  the  cuts. 

In  the  handling  of  areas  or  spots  in  an  orchard  where  Citrus  trees 
are  affected  or  killed  out,  it  is  necessary  not  only  to  check  the  out- 
ward advance  of  the  fungus  so  that  new  trees  will  not  become  infected, 
but  to  treat  the  area  already  affected  before  replanting.48    To  prevent 


47  Home,   W.   T.,   Calif.    State   Comm.    of   Hort.    Mo.    Bull.    3:275-282,    1914: 
4:179,  1915. 

48  Barrett,  J.  T.,  Calif.  Citrograph,  3:77-78,  1918. 


Bull.  395]       BARK   DISEASES   OF    CITRUS   TREES   IN    CALIFORNIA  31 

new  trees  from  becoming  infected,  Professor  Home  suggests  digging 
ditches  so  as  to  inclose  the  affected  areas.  For  killing  out  the  fungus 
in  the  soil  of  the  affected  area  before  replanting,  IV2  ounces  of  carbon 
bisulphid  poured  into  holes  1%  feet  apart  each  way  and  1%  feet 
deep  has  been  used  usually  with  good  results. 


BOTRYTIS  GUMMOSIS 

SYMPTOMS  AND  OCCUEEENCE 

Botrytis  gummosis40  differs  from  brown -rot  (Pythiaeystis)  gum- 
mosis, in  that  it  causes  softening  of  the  invaded  bark  in  the  early 
stages  and  shows  a  gray  color  on  the  surface  in  damp,  cool  weather, 
caused  by  the  conidiophores  and  spores  of  the  fungus  (fig.  8).  In 
the  later  stages  the  outer  layer  of  bark  is  killed  and  becomes  dry  and 
hard  much  in  advance  of  the  inner  layer,  while  there  is  a  greater 
tendency  than  in  brown-rot  gummosis  for  the  tree  to  renew  the  bark 
underneath  the  dead,  hard  layer,  and  there  is  usually  also  a  less 
copious  flow  of  gum.  Unlike  brown-rot  gummosis,  Botrytis  gummosis 
is  confined  in  California  almost  exclusively  to  lemon  trees  growing 
in  the  coastal  regions,  and  usually  occurs  on  trees  that  are  more  than 
10  years  of  age.  This  disease  should  not  be  confused  with  "shell 
bark,"  a  desquamated  bark  condition  in  which  the  outer  bark  of 
lemon  trees  dies,  cracks  and  breaks  away  in  longitudinal  strips,  a 
condition  which  is  somewhat  similar  to  that  frequently  brought  about 
in  the  later  stages  of  Botrytis  gummosis.  These  two  diseases  are 
often  associated  on  the  same  trees.  The  conditions  favorable  to  the 
one  are  also  apt  to  encourage  the  other.  Neither  disease  should  be 
confused  with  psorosis    (scaly  bark)    of  sweet-orange  trees. 

Unlike  Pythiaeystis  citrophthora,  the  causative  fungus,  Botrytis 
cinerea,  is  not  able  to  gain  entrance  except  through  some  wound  or 
defect  in  the  bark,  and  is  not  able  to  progress  so  rapidly  in  killing 
the  bark  through  to  the  wood.  A  large  area  is  involved,  in  which 
only  certain  outer  layers  of  bark  tissue  are  killed,  leaving  the  cam- 
bium alive  and  capable  of  renewal.  This  gummosis  produces  an 
outer  gummous  zone  beyond  the  invaded  area,  but  this  is  usually  less 
extensive  and  less  rapidly  formed  than  with  Pythiaeystis  gummosis. 
Other  conditions  being  equal,  there  is  usually  somewhat  less  gum 
formation  in  Botrytis  gummosis  than  in  Pythiaeystis  gummosis. 


49  Fawcett,  H.  S.,  "Two  fungi  as  causal  agents  in  gummosis  of  lemon  trees 
in  California,"  in  Calif.  Comm.  Hort.  Mo.  Bull.  2:601-617,  1913,  and  Phyto- 
pathology, 4:54,  1914. 


32  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

My  attention  was  first  called  to  this  type  of  gummosis  early  in 
February,  1912.  After  a  period  of  moist,  cool,  weather,  patches 
of  bark  6  to  12  inches  long  and  half  as  wide  presented  the  gray  furry 
appearance  characteristic  of  the  fruiting  bodies  of  Botrytis  cinerea 
(fig.  8).  In  a  later  survey  of  the  Citrus  districts  of  California, 
Botrytis  cinerea  was  always  found  associated  with  this  type  of  gum- 
mosis and  was  isolated  from  a  large  number  of  diseased  trees. 

INVESTIGATIONS  AS  TO  NATURE  AND  CAUSE 

The  investigations  establishing  the  relation  of  the  fungus,  Botrytis 
cinerea,  to  this  disease  were  conducted  in  much  the  same  way  as  those 
in  connection  with  brown-rot  gummosis. 

(1)  Inoculation  with  diseased  tissue  showed  that  the  disease  could 
be  transmitted  to  sound  bark  of  healthy  trees. 

(2)  Cultures  from  the  diseased  lesions  showed  the  presence  of  a 
fungus,  Botrytis  cinerea,  previously  known  as  the  Botrytis  rot  fungus 
of  the  packing  houses. 

(3)  By  inoculation  experiments  with  pure  cultures  of  this  fungus 
the  characteristic  symptoms  were  induced  on  healthy  trees   (fig.  9). 

(4)  Inoculation  with  Botrytis  fungus  isolated  from  lemon  fruits 
also  produced  the  same  results. 

(5)  The  Botrytis  fungus  was  again  isolated  from  the  artificially 
induced  lesions  and  was  found  capable  of  inducing  rot  in  lemon  fruits 
irrespective  of  whether  it  had  been  found  originally  in  diseased  bark 
or  in  rotting  fruit.  Some  of  the  details  of  this  work  are  published 
elsewhere.50 

The  fungus  was  isolated  from  the  softened  invaded  area  of  a  large 
number  of  these  lesions.  Attempts  to  isolate  the  fungus  from  the 
outer  gummous  zone,  however,  failed,  just  as  they  did  in  Pythiacystis 
gummosis.  Only  rarely  was  Botrytis  isolated  from  the  area  where 
the  outer  bark  was  dead  and  hard.  Cultures  showed  that  after 
Botrytis  infection  this  outer  dead  cortical  layer  is  rapidly  occupied 
under  such  conditions  as  prevail  at  Santa  Paula  by  species  of  Alter- 
naria,  Cladosporium,  Penicillium,  Colletotrichum,  Fusarium,  and 
other  fungi  and  bacteria. 


so  Gummosis  of  Citrus.  Jour.  Agri.  Res.  24:191-236,  1923. 


Bull.  395]       BARK   DISEASES   OF    CITRUS   TREES   IN    CALIFORNIA 


33 


Fig.  8. — Gray  masses  of  spoies  of  Botrytis  cinerea  on  the  surface  of  bark 
of  a  Botrytis  gummosis  lesion. 


34  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


FACTOKS  FAVOKING  THE  DISEASE 

Many  contributing  conditions  tend  to  favor  the  occurrence  and 
the  severity  of  this  disease.  Some  of  these  are  similar  to  those  which 
favor  Pythiacystis  gummosis. 

Injuries  of  various  kinds  to  the  bark,  not  only  near  the  soil,  but 
anywhere  on  the  trunk  or  large  branches,  may  lead  the  way  to  infec- 
tion and  development  of  Botrytis  gummosis  when  the  conditions  Of 
moisture  and  temperature  are  also  favorable.  This  disease  is  fre- 
quently severe  on  living  tissue  of  trees  that  have  been  injured  by 
frost.  The  fungus  may  first  become  established  in  such  trees  in  a 
small  portion  of  dead  or  dying  tissue  and  then  advance  rapidly  into 
tissue  which  appears  to  be  sound. 

A  desquamated  condition  of  bark,  known  as  decorticosis,  is  fre- 
quently associated  with  Botrytis  gummosis.  It  furnishes  dead  outer 
bark  tissue  from  which  the  fungus  may  advance.  Decorticosis  is 
similar  in  appearance  to  the  effect  which  often  follows  inoculation 
with  Botrytis  cinerca  on  sound  tree  trunks  and  with  which  it  is 
often  confused. 

The  previous  use  of  neat's  foot  oil  in  the  treatment  of  gummosis 
encouraged  the  growth  of  the  Botrytis  fungus.  The  trunks  of  lemon 
trees  treated  by  scoring  the  bark  and  painting  with  neat's  foot  oil 
were  observed  in  February  and  March  of  1912  to  be  fairly  well 
covered  with  a  gray  coating  consisting  of  the  sporophores  and  spores 
of  Botrytis.  The  bark  on  these  trees  was  found  to  be  in  various  stages 
of  soft  decay  with  the  exudation  of  large  masses  of  gum.  Experi- 
ments also  showed  that  this  fungus  develops  better  on  lemon  bark 
treated  with  neat's  foot  oil  either  before  or  after  infection  by  the 
organism  than  on  bark  free  from  this  oil.  In  recent  years  the  appli- 
cation of  neat's  foot  oil  to  Citrus  trees  has  been  largely  given  up, 
and  the  more  severe  stages  of  this  disease,  such  as  were  previously 
seen,  have  not  been  observed  lately. 

METHODS  OF  CONTEOL 

The  control  methods  used  for  Botrytis  gummosis  as  in  the  case 
of  Pythiacystis  gummosis  are  of  two  kinds,  preventive  and  curative ; 
both  of  which  are  similar  in  principle  to  those  discussed  in  connection 
with  the  former  disease.  A  few  modifications,  however,  based  on  the 
differences  between  the  two  diseases  should  be  pointed  out. 

Prevention. — Since  Botrytis  einerea  appears  to  be  dependent  upon 
abrasions  or  other  injuries  for  its  entrance  into  lemon  bark,  especial 


Bull.  395] 


BARK    DISEASES    OF    CITRUS    TREES    IN    CALIFORNIA 


35 


care  is  necessary,  particularly  in  moist  weather,  to  avoid  injuries  in 
cultivation,  picking  and  other  orchard  operations.  The  danger  from 
such  injuries  may  be  lessened  by  painting  the  tree  trunks  with  Bor- 
deaux paste  or  other  fungicides  or  spraying  them  thoroughly  with 
Bordeaux  mixture.  The  precaution  previously  mentioned  of  pulling 
away  soil  that  is  too  high  against  the  trunk  and  of  keeping  water  as 
much  as  possible  away  from  the  trunk,  are  also  applicable  in  the 
case  of  Botrytis  gummosis. 


Fig.  9.— (a)  Gum  exudation  on  a  lemon  trunk  as  the  result  of  inoculation 
with  the  Botrytis  fungus. 

(b)  Bark  over  and  around  the  diseased  lesion  scraped,  to  show  the  extent 
of  the  dead  soft  bark. 


Treatment. — The  principle  governing  the  treatment  of  this  type 
of  gum  disease  is  the  same  as  that  for  Pythiacystis  gummosis,  namely, 
the  elimination  of  the  invaded  tissue  and  the  prevention  of  further 
progress  of  the  disease.  As  the  result  of  many  different  experiments 
in  which  growers  took  a  prominent  part,  a  method  consisting  largely 
of  scraping  off  the  outermost  layers  of  bark  was  found  which  proved 
to  be  best  adapted  for  treatment  of  this  disease  (fig.  10).  The  portion 
where  the  bark  is  totally  killed  is  cut  away,  but  beyond  this  where 
usually  only  the  outer  layers  of  bark  are  dead,  these  outer  layers  only 


36 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


are  scraped  off,  leaving  intact  the  live  inner  layer  next  to  the  cam- 
bium. To  prevent  further  invasion  of  the  outer  bark  it  is  usually 
found  necessary  to  scrape  the  sound  bark  several  inches  beyond  the 
margin  of  the  affected  region.    A  sharp  curved  tool,  described  under 


Fig.  10. — Treatment  of  Botrytis  gummosis. 

(a)  Lesion  being  scraped. 

(b)  Painting   scraped  area  with   Bordeaux  paste. 


"psorosis, "  which  can  be  easily  controlled  in  scraping,  is  in  general 
use  for  this  purpose  (fig.  15).  This  modification  of  the  method  used 
for  Pythiacystis  gummosis  is  advisable  because  of  the  different  nature 
of  the  disease,  in  order  to  preserve  as  much  as  possible  of  the  inner 
live  bark.     Where  both  types  of  gummosis  are  present  on  the  same 


BULL.  395]       BARK   DISEASES   OF    CITRUS   TREES   IN    CALIFORNIA  37 

trees,  as  is  frequently  the  case,  this  method  is  still  applicable  to  the 
combined  lesions  produced.  The  cut  or  scraped  portions  are  then 
painted  with  a  fungicide  (fig-.  10b).  Bordeaux  paste,  and  some  of 
the  coal  tar  products,  such  as  Avenarius  Carbolineum,  and  Arrow 
Carbolineum,  which  contain  only  the  heavier  oils,  have  given  good 
results. 

The  following  is  a  typical  example  of  the  many  experiments  in 
treatment  of  this  disease : 

On  October  8,  1912,  at  Santa  Paula,  a  lesion  on  the  trunk  of  an 
18-year-old  lemon  tree  was  treated  by  cutting  away  the  bark  entirely 
in  a  few  small  places  where  it  was  killed  to  the  wood,  but  scraping 
away  only  the  dead  outer  layers  elsewhere  and  leaving  the  cambium 
still  attached  (fig.  10a).  The  scraping  was  done  with  a  sharp  curved 
instrument  made  on  the  principle  of  a  box  scraper,  and  the  scraped 
portion  was  then  painted  with  Bordeaux  paste  (fig.  105). 

On  May  10,  1913,  new  bark  was  building  up  over  the  scraped 
portion,  but  the  disease  had  spread  slightly  on  the  margins  where 
scraping  had  not  been  continued  out  far  enough.  It  was  then  scraped 
again. 

On  May  17,  1914,  new  bark  was  seen  to  have  formed  over  the 
entire  scraped  portion  and  the  disease  appeared  to  be  completely 
checked. 


SCLEROTINIA  GUMMOSIS  DUE  TO  SCLEROTINIA 
LIBERTIANA 

This  disease,  usually  of  minor  importance,  occasionally  has  been 
found  associated  with  rapid  dying  of  bark  on  the  roots  and  trunks  of 
Citrus  trees  growing  in  damp,  cool  locations  especially  after  periods 
of  severe  frost.  The  bark  is  at  first  soft  just  as  in  the  case  of  attack 
by  Botrytis  cincrea,  Though  this  fungus  usually  advances  more 
rapidly  than  Botrytis,  it  is  soon  checked,  and  callus  begins  to  form 
as  soon  as  gum  accumulates.  Later,  as  the  bark  dries,  it  is  left  in 
shreds  (fig.  11)  and  large  black  sclerotia  are  found  within  and  under 
this  bark.  Its  effect  on  twigs  has  been  described  by  C.  0.  Smith,51 
who  refers  to  the  gumming  usually  accompanying  its  attack.  It 
appears  to  infect  the  young  growth,  usually  at  the  blossoming  period, 
and  frequently  extends  back  into  larger  branches. 

When  the  fungus  is  found  on  the  trunk  or  roots,  observations 
have  indicated  that  previous  injury  of  the  bark  was  usually  necessary 


si  Smith,  C.  O.,  Cottony  rot  of  lemons  in  California.     Calif.   Agr.   Exp.   Sta. 
Bull.  265:237-258,  1916. 


38 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


for  its  entrance.  It  has  frequently  been  found  on  young  trees  after 
frost  injuries,  apparently  advancing  from  frost  injured  tissue  into 
tissue  not  killed  b}^  frost.     It  has  been  observed  on  a  lemon  tree  20 


Fig.  11.— Boot  of  lemon  tree  showing  shredded  appearance  of  the  bark  and 
the  black  sclerotia  as  the  result  of  the  attack  of  Sclerotinia  libcrtiani.  Arrows 
point  to  sclerotia. 

years  old  where  all  the  roots  had  been  infected,  probably  at  wounds 
made  in  digging  about  them  and  placing  vetch  straw  near  the  crown 
in  damp  cool  weather.  An  old  seedling  orange  tree  also  was  observed 
with  the  bark  on  one  side  of  the  trunk  killed  by  the  fungus,  which 


BULL.  395]       BARK   DISEASES   OF    CITRUS   TREES   IN    CALIFORNIA  89 

had  apparently  gained  entrance  through  a  small  sunburned  area  and 
thence  advanced  into  the  live  bark  for  some  distance. 

This  fungus  has  also  been  shown  by  inoculation  experiments  to  be 
capable  of  advancing  rapidly  into  lemon  bark,  inducing  copious  gum 
formation  for  a  short  time  only.  Observation  shows  that  the  halting 
of  the  invasion  of  the  fungus  is  usually  coincident  with  the  exudation 
of  considerable  quantities  of  gum.  The  prevention  and  treatment  of 
this  disease  is  the  same  as  that  for  Botrytis  gummosis,  (See  direc- 
tions under  Botrytis  gummosis,  page  35.) 


PSOROSIS  (SCALY  BARK)  OF  ORANGE  TREES 

Although  psorosis  is  generally  classed  as  a  gum  disease,  gum 
exudes  only  at  certain  seasons,  or  during  certain  phases  of  develop- 
ment of  the  disease,  and  then  the  gum  is  usually  not  so  conspicuous  a 
feature  as  it  is  with  brown-rot  gummosis. 

This  disease  was  briefly  described  by  W.  T.  Swingle  and  H.  J. 
Webber  in  1896  in  Florida  and  given  the  name  of  psorosis. 

It  is  now  known  in  California  as  "scaly  bark,"  but  must  not  be 
confused  with  another  disease  known  in  Florida  as  scaly  bark  (nail- 
head  rust).  This  is  distinct  from  psorosis  and  does  not  occur  in 
California. 

The  slowness  of  the  development  of  the  disease,  and  its  inconspicu- 
ous appearance  and  lack  of  effect  upon  the  foliage  in  the  earlier 
stages,  frequently  prevent  it  from  being  noticed  until  it  is  far 
advanced. 

SYMPTOMS 

The  most  conspicuous  feature  of  the  scaly-bark  disease  is  the 
appearance  on  the  trunk  or  large  limbs  of  irregular  scales  of  bark 
14  to  1  inch  in  diameter,  standing  out  as  if  pushed  off  from  the 
surface  (fig.  13).  It  usually  begins  Avith  a  very  small  area  in  which 
only  a  thin  outer  layer  of  bark  dies,  hardens,  and  is  raised  from  the 
surface  (fig".  12),  leaving  a  layer  of  bark  underneath  still  alive.  This 
first  area  slowly  enlarges  from  year  to  year  until  finally  it  encircles 
the  trunk  or  limb  (fig.  14a).  Often  a  number  of  small  areas  begin 
at  the  same  time  and  thus  cover  the  surface  more  rapidly.  Later 
the  deeper  layers  of  bark  and  even  the  wood  may  be  affected.  Gum 
may  exude  as  the  disease  advances,  but  gum  is  not  a  necessary  accom- 
paniment of  the  disease  and  its  presence  and  amount  depend  on 
growth  conditions,  season  of  the  year,  etc. 


40 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


INVESTIGATIONS  AS  TO  CAUSE  AND  MANNER  OF  DEVELOPMENT 

An  investigation  into  the  cause  and  manner  of  development  and 
the  control  of  this  disease,  has  been  carried  on  for  years.  Many 
difficulties  which  were  not  encountered  in  the  work  with  brown-rot 
gummosis  have  arisen  in  the  investigation  of  the  cause  of  psorosis. 
One  of  these  has  been  the  extreme  slowness  with  which  the  disease 
develops  in  its  incipient  stages. 


;*■•■•■..■.■•■.=:  •; 

f '  *  .it     **■  ^ 


Fig.  12. — Small  lesions  of  psorosis   (scaly  bark)  ;   one  showing  bark  breaking 
away  in  scales;   others  above  showing  a  roughness  of  bark.     Slightly  magnified. 


In  certain  cases  it  has  been  possible  to  transmit  the  disease  to 
sound  trees  by  inoculations  with  bits  of  tissue  from  diseased  lesions, 
but  many  of  the  attempts  have  failed.  In  one  of  the  successful  cases 
two  years  elapsed  after  the  diseased  tissue  was  placed  in  a  wound  in 
sound  bark,  before  any  sign  of  the  disease  was  noted.  This  suggests 
that  an  organism  of  some  kind  which  is  able  to  advance  very  slowly 


BULL.  395]       BARK   DISEASES   OF    CITRUS   TREES   IN    CALIFORNIA 


41 


may  be  the  immediate  cause  of  the  disease.  Experiments  planned  to 
determine  whether  this  hypothesis  is  correct  have  been  under  way  for 
some  time,  but  because  of  the  slowness  of  development  of  psorosis 
from  the  early  stages  there  has  not  been  time  for  proof  of  this  con- 
jecture to  be  established. 


Fig.  13. — About  the  second  stage  of  psorosis  (scaly  bark)  on  orange  trees, 
showing  formation  of  scales  of  bark,  giving  the  surface  a  roughened  scaly 
appearance. 

As  examples  of  the  slowness  with  which  the  disease  often  develops, 
the  following  cases  may  be  mentioned.  At  the  Experiment  Station 
two  incipient  lesions  not  yet  gumming,  and  each  about  1  inch  in 
diameter  were  outlined  on  a  Valencia  orange  tree  12  years  old.  These 
lesions  had  progressed  only  %  inch  beyond  the  original  mark  in  2% 
years.     A  slightly  larger  area  on  another  tree,  which  was  1%  inches 


42  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

in  diameter  when  first  observed  became  only  4  inches  in  diameter  in 
3%  years.  These  areas  were  so  small  and  inconspicuous  when  out- 
lined that  they  would  not  have  been  noted  at  all  at  that  stage  by 
most  growers. 

After  the  lesions  become  larger,  however,  they  develop  somewhat 
more  rapidly.  The  following  example  represents  a  rather  rapidly 
developing  lesion.  An  area  on  a  trunk  of  a  12-year-old  Valencia  tree 
was  3x7  inches  when  marked.  Six  months  later  it  was  8x12  inches; 
after  9  months,  10x15  inches;  after  2  years,  12x16  inches  (with  much 
gumming),  and  after  3%  years  14x17  inches.  While  this  increase 
from  3x7  inches  to  14x17  inches  seems  considerable  in  itself,  it  repre- 
sents an  average  of  only  about  1%  inches  of  advance  per  year  in  any 
one  direction.  The  lesion  was  probably  3  to  4  years  old  when  first 
measured.  Scaly  bark  thus  differs  from  any  other  gum  diseases, 
whose  whole  course  of  development  can  be  followed  out  in  1  or  2 
years. 

The  visible  advance  of  the  disease  depends  greatly  upon  the  season 
of  the  year.  In  the  majority  of  cases  it  seems  to  be  most  active  during 
the  growing  season  and  especially  in  summer  and  early  fall,  and  to 
be  quiescent  during  the  winter  and  early  spring.  This  summer 
increase  of  activity  is  due  in  part  to  the  breaking  away  of  scales  of 
bark,  probably  brought  about  not  only  by  drying  of  the  outer  bark 
that  has  previously  died,  but  also  by  the  increase  in  growth  of  the 
inner  live  bark  beneath.  Gum  formation  and  exudation  also  take 
place  at  this  season,  giving  further  indication  of  activity.  Most  of 
the  advance  for  the  year,  however,  appears  to  be  finished  when  the 
gum  formation  is  at  its  maximum.  The  gum  is  the  result  and  not 
the  cause  of  the  death  of  the  outer  bark.  The  gum  probably  tends  to 
hinder  more  than  to  aid  the  advance  of  the  disease  at  this  time. 

EXPERIMENTS  IN   TREATMENT 

Although  the  investigation  as  to  the  cause  and  manner  of  develop- 
ment of  the  disease  received  first  attention,  a  number  of  experiments 
in  control  have  also  been  carried  on.  Various  methods  of  cutting, 
scraping  and  slitting  the  bark  have  been  compared  with  no  cutting  or 
scraping.  Comparisons  have  also  been  made  of  the  application  of  a 
considerable  number  of  substances  as  fungicides  or  coverings. 

Without  going  into  detail,  it  may  be  said  that  a  method  of  scrap- 
ing the  outer  bark  to  depths  varying  according  to  circumstances,  to 
be  described  later,  has  been  found  more  effective  than  any  other 
method  (fig.  14).    Scraping  has  been  found  to  be  much  more  import- 


BULL.  395]       BARK   DISEASES   OF    CITRUS   TREES   IN    CALIFORNIA 


43 


ant  than  the  use  of  any  kind  of  fungicide  or  disinfectant.  Slitting 
has  proven  of  little  or  no  value  in  these  experiments.  These  experi- 
ments have  also  shown  the  advisability  of  scraping  not  only  the  area 
visibly  affected,  but  also  beyond  the  area  to  a  distance  of  from  6  to  8 


Fig.  14. — A.  Psorosis  second  stage  on  Valencia  orange  trunk.  Area  had 
previously  been  whitewashed  but  not  scraped.  B.  Same  tree  after  scraping 
outer  layers  of  bark  for  about  six  inches  beyond  the  diseased  area.  Scraping 
a  little  deeper  at  the  advancing  edge  than  here  shown  is  advisable  for  ideal 
treatment. 


inches  to  head  off  the  advance  of  the  disease  in  bark  not  yet  visibly 
affected.  The  spread  of  a  lesion  in  its  early  stages  appears  to  take 
place  within  the  outermost  layers  of  bark.  This  accounts  for  the  bene- 
ficial results  of  light  scraping  beyond  the  visible  edge  of  the  lesions. 


44  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

A  large  number  of  disinfectants  and  coverings  were  tested  with 
and  without  scraping  or  other  method  of  cutting  the  bark.  In  each 
case  similar  areas  on  the  same  or  different  trees  were  left  without  the 
application  of  the  disinfectant  for  comparison.  Almost  without 
exception  the  diseased  trees  on  which  the  bark  was  carefully  scraped 
showed  the  best  recovery.  The  disinfectant  applied  seemed  to  have 
little  effect.  Lesions  of  the  same  kind  not  scraped  continued  to 
develop  in  size  at  the  same  rate,  whether  painted  or  unpainted  with 
the  various  disinfectants,52 

The  results  of  experiments  obtained  so  far  indicate  that  the  par- 
ticular fungicide  applied  is  of  less  importance  than  the  manner  of 
scraping  or  treating  the  bark. 


SUGGESTIONS  FOR  TREATMENT 

The  stage  of  the  disease  largely  determines  how  the  tree  should  be 
handled.  Each  tree  affected  presents  an  individual  problem.  The 
general  suggestions  made  here  apply  to  typical  conditions  and  may 
need  modification  for  particular  cases. 

First  Stage. — At  the  very  beginning  (fig.  12)  only  an  outer  layer 
of  bark  appears  to  be  injured  or  changed,  an  inner  layer  next  to  the 
cambium  still  being  alive  and  active  and  free  from  discoloration 
except  that  it  frequently  presents  a  slight  greenish  appearance. 
Later  a  yellowish  discoloration  may  appear,  due  to  the  development 
of  gum  within  the  tissue.  The  trunk  and  large  limbs  of  all  trees 
should  be  inspected  carefully  to  detect  the  disease  at  its  very  begin- 
ning. When  the  beginning  areas  are  small  and  do  not  cover  more 
than  %  of  the  circumference,  the  affected  bark  may  be  scraped  rather 
deeply  and  the  surrounding  apparently  unaffected  bark  scraped  less 
deeply  for  4  to  6  inches  in  all  directions  beyond  the  margin  of  the 
affected  areas, 

Second  Stage. — When  the  disease  has  progressed  further,  so  as 
to  cover  more  than  about  %  of  the  circumference  of  the  entire  bark 
on  the  trunk  but  has  not  yet  seriously  injured  the  wood,  it  may  for 
convenience  be  considered  to  be  in  the  second  stage.  At  this  stage 
the  affected  portion  often  presents  a  roughened  surface  (figs.  13  and 
14). 

Fresh  scaling  of  the  bark  on  the  advancing  edges  continues  and 
exudation  of  gum  takes  place  at  certain  seasons  of  the  year.  In  such 
cases  the   cure  is  more  uncertain,   but  the   disease  may  be   checked 


-2  Rounds,  M.  B.,  Ann.  Rept.  Calif.  Citrus  Institute,  1921-22,  p.  54-60. 


BULL.  395]       BARK   DISEASES   OF   CITRUS   TREES   IN    CALIFORNIA  45 

and  sometimes  cured  by  a  light  to  medium  scraping.  Particular 
attention  should  be  given  to  the  advancing  edges.  The  affected 
surface  may  then  be  treated  with  one  of  the  fungicides  mentioned 
later.  Six  months  or  a  year  later,  these  should  be  treated  again, 
scraping  only  where  the  disease  is  still  active.  The  progress  of  the 
disease  is  so  slow  that  usually  one  cannot  discern  within  less  than  six 
months  to  a  year  or  more,  whether  it  has  progressed  or  not. 

Third  Stage. — Where  the  disease  has  been  present  for  a  number 
of  years  (5  to  10  or  more)  or  until  a  greater  part  of  the  bark  of  the 
trunk  is  affected  and  the  wood  underneath  is  killed  and  beginning  to 
decay,  there  is  little  hope  for  a  permanent  recovery.  A  tree  of  this 
kind,  however,  sometimes  remains  surprisingly  productive  for  a  num- 
ber of  years,  so  that  it  becomes  a  question  whether  to  replace  it  at 
once  or  to  treat  it  superficially  with  the  idea  of  preventing  possible 
spread  to  other  trees,  and  of  taking  it  out  later.  If  only  a  part  of  the 
branches  show  the  disease  in  the  second  and  third  stages,  these  may 
be  cut  out  entirely  and  the  remaining  part  of  the  tree  frequently 
inspected  for  further  outbreaks.  If  the  wood  is  just  beginning  to  be 
discolored  and  killed,  this  should  be  chisled  out  and  benzene-asphalt 
paint,  white  lead  or  other  good  wood  protecting  covering  applied  to 
the  exposed  surfaces.  A  severe  cutting  back  of  a  badly  affected  tree 
may  be  helpful.  If  the  trunk  is  too  badly  decayed  and  the  entire  tree 
appears  stunted  and  unproductive,  the  tree  should  be  dug  out. 

Season  of  the  Year. — Where  there  is  much  frost  hazard,  bark 
scraped  too  late  in  the  fall  or  during  the  winter  months  is  likely  to  be 
killed  by  low  temperatures.  Experiments  at  Riverside  indicate  that 
bark  treated  in  the  late  spring  and  summer  months  recovers  most 
rapidly. 

Fungicides. — As  was  previously  stated,  the  kind  of  fungicide 
employed  appeared  to  be  of  minor  importance  in  certain  of  our 
experiments,  Some  kind  should  be  applied,  however,  to  the  scraped 
areas,  as  a  matter  of  precaution.  Except  for  the  slight  danger  to 
.the  foliage  when  followed  by  fumigation  there  is  nothing  better  than 
Bordeaux  paste.  Other  substances  of  good  fungicidal  value  are 
some  of  the  high-boiling  coal  tar  products  such  as  Arrow  Carbolineum 
and  Avenarius  Carbolineum. 

Other  safe  fungicides  of  known  composition  are:  mercuric  chlo- 
ride 1  gram  to  1000  cc  of  25  per  cent  denatured  alcohol  (%  ounce 
to  one  gallon)  or  mercuric  cyanide  of  the  same  strength;  emulsified 
cresol  (liquor  cresolis  compositus  USP)  10  per  cent  in  water;  or 
potassium  permanganate  crystals  1  per  cent  in  water  (one  ounce 
in  3  quarts  of  water).     The  mercuric  chloride  and  mercuric  cyanide 


46 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


may  be  obtained  as  tablets  or  in  crystalline  form,  the  potassium  per- 
manganate and  copper  sulphate  in  crystalline  form,  the  liquor  cre- 
solis  compositus  and  carbolineums  in  liquid  form.  These  are  sold 
by  drug  stores,  drug  companies  and  spray  dealers, 

Before  applying  to  the  bark  carbolin- 
eum  or  any  other  product  the  exact  com- 
position or  effect  of  which  is  unknown,  it 
should  be  tested  on  some  leaves  and  twigs 
several  days  before  it  is  to  be  used  on  the 
bark.  If  in  two  or  three  days  it  does  not 
seriously  injure  the  nature  leaves  or  twigs 
it  will  probably  be  safe  to  apply  to  the 
bark.  If,  however,  the  leaves  are  severely 
burned,  the  product  is  usually  not  safe  to 
use  on  the  bark. 

For  Bordeaux  paste,  dissolve  1  pound  of 
bluestone  (copper  sulfate)  in  3  quarts  of 
water  in  a  wooden,  earthen  or  glass  vessel 
and  slake  2  pounds  of  lime  in  3  quarts  of 
water.  The  bluestone  is  most  easily  dis- 
solved by  suspending  it  in  a  sack  at  the  top 
of  the  water  overnight.  If  the  bluestone  is 
pulverized  and  suspended  in  warm  water 
it  dissolves  rapidly.  Good  lime  that  is  not 
air-slacked  should  be  used.  If  covered  to 
avoid  evaporation  the  dissolved  ingredients 
will  keep  indefinitely  in  separate  vessels. 
Where  the  paste  is  being  used  over  a  num- 
ber of  days  or  weeks,  just  enough  of  the  wet 
slaked  lime  and  the  bluestone  solution 
should  be  mixed  to  last  for  one  or  two  days. 
It  may  be  applied  with  large  whitewash 
brushes.  Commercial  Bordeaux  pastes  and 
powders  brought  to  equivalent  strength 
may  also  be  used.  Most  of  these  will  require 
the  addition  of  some  lime  paste. 

Tools. — A  number  of  different  kinds  of 

tj,;     -.  _     ^    ,  -  scrapers  are  in  use  for  scraping  the  bark. 

Fig.  lo. — Tool  for  scrap-  ^  ^     to 

ing  the  bark  or  for  goug-  Several  types  devised  by  Mr.  Culbertson  of 

ing    out     small     diseased  ^     Limoneira    Company   for    use   in   the 
areas    caused    by    psorosis  r      ^ 

or  other  bark  diseases.  treatment  of  Botrytis  gummosis  are  shown 


BULL.  395]       BARK   DISEASES   OF    CITRUS   TREES   IN    CALIFORNIA  47 

in  figure  10.  A  modification  of  one  of  these,  first  made  under  the 
direction  of  Dr.  J.  T.  Barrett  of  the  Citrus  Experiment  Station,  is 
now  in  common  use.  It  may  be  made  of  spring  steel  and  consists  of 
a  curved  blade  sharpened  on  both  edges  and  at  the  end  and  set  in 
a  wooden  handle.  The  handle  is  l1/^  inches  long  and  l1/!  inches  in 
diameter  and  the  steel  part,  including  the  curved  blade,  is  5%  inches 
long  (fig.  15).  These  can  be  made  by  a  local  blacksmith  or  obtained 
through  the  farm  advisor. 


DECORTICOSIS  OR  SHELL  BARK  OP  LEMON  TREES 

Decorticosis  (shell  bark)  is  a  disease  affecting  the  outer  layers  of 
the  bark  of  lemon  trees  principally  that  of  the  trunks.  In  some 
instances,  the  disease  spreads  up  on  to  the  basal  portion  of  the  larger 
limbs,  but  in  general  it  is  confined  to  the  bark  of  the  trunk.  It  is 
not  known  to  affect  anything  except  the  lemon  bark.  The  general 
aspect  of  decorticosis  recalls  the  loosening  bark  of  shell  bark  hickory 
tree  of  the  eastern  states.  This  resemblance,  spoken  of  by  Fawcett53 
in  describing  the  disease,  led  Coit  and  Blanchard54  to  employ  the 
term  "shell  bark." 

Shell  bark  is  frequently  confused  with  psorosis  of  orange  trees, 
a  very  different  disease  which  never  spreads  to  the  lemon  bark. 

Distribution  and  History. — It  is  not  known  how  widely  this  dis- 
ease occurs  in  various  countries  which  grow  lemons.  It  does  uot 
appear  to  have  attracted  any  attention  in  California  until  recently. 
It  was  already  fairly  common  in  many  orchards,  however,  in  1912, 
when  my  investigation  of  Citrus  diseases  in  this  state  began,  but 
no  definite  published  mention  of  the  disease  had  been  seen  at  that 
time.  Lemon  tree  trunks  showing  similar  symptoms  were  seen  in 
Florida  and  Cuba  in  191453  and  in  Sicily  in  1923. 55  It  is  probable 
that  it  has  been  in  existence  for  a  long  time  and  may  be  widely  dis- 
tributed. A  species  of  fungus,  Phoniopsis  californica,  has  been 
shown  to  play  a  part  in  the  development  of  the  disease.50 


53  Fawcett,  H.  S.,  Citrus  diseases  of  Florida  and  Cuba  compared  with  those 
of  California.  Calif.  Agri.  Exp.  Sta.  Bull.  262:149-210,  1915. 

54  Coit,  J.  E.,  and  Blanchard,  V.  F.     Calif.  Citrog.  7:259-292,  1922. 

ss  Fawcett,  H.  S.,  Observations  on  bark  diseases  of  Citrus  trees  in  Sicily. 
Phyto.   15:41-42,   1925. 

56  Fawcett,  H.  S.  A  new  Phomopsis  of  Citrus  in  California.  Phyto.  12:419- 
424,  1922;  Shell  bark  of  lemon  trees,  Calif.  Citrograph,  9:330,  1924. 


48 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


Symptoms. — Shell  bark  is  found  only  on  lemon  trees  or  on  trees 
having  lemon  trunks.  The  outer  bark  dies,  shrinks  and  cracks  and 
(in  advanced  cases,  fig.  16)  tends  to  loosen  in  vertical  strips.  The 
drying  and  cracking  of  the  outer  bark  frequently  starts  at  the  bud 
union   near   the   soil   and   gradually   works   upward   till   the   entire 

trunk  is  involved  (fig.  16).  In 
pure  shell  bark  that  is  not  accom- 
panied by  gum  diseases,  the 
inner  bark  and  cambium  remain 
alive  and  tend  to  build  up  new 
layers  to  replace  the  outer  de- 
stroyed layer.  Young  bark  ap- 
pears to  be  immune  to  the 
disease,  it  being  found  only  in 
connection  with  bark  of  a  con- 
siderable age. 

As  a  rule  the  disease  does  not 
appear  until  a  lemon  tree  is  from 
15  to  20  years  of  age.  With 
some  varieties,  as  the  Lisbon,  the 
age  at  which  the  disease  starts 
may  be  20  to  30  years.  Some 
susceptible  strains  of  Eureka, 
however,  may  begin  to  be  affected 
when  only  10  to  12  years  old. 

The  general  effect  on  the  foli- 
age and  appearance  of  the  tree 
may  be  quite  marked  when  shell 
bark  is  advancing  rapidly  in  the 
bark  tissues.  The  leaves  gradu- 
ally turn  a  bronze  color,  some  of 
them  fall  prematurely,  leaving 
a  rather  sparse  foliage  or  bare 
twigs,  the  tips  of  which  may  die 
back  for  a  short  distance.  In  less 
severe  cases  there  may  be  only  an 
indefinite  "off  color"  and  lack 
of  vigor  without  any  characteristic  symptoms  in  the  top  of  the  tree. 
After  the  entire  outer  bark  of  the  trunk  has  become  involved  and 
begins  to  crack  and  loosen  in  strips  and  the  inner  bark  begins  to 
build  up  again,  the  foliage  may  wholly  or  partially  recover  its  former 
green   vigorous   appearance.        In   certain   Eureka  lemon   orchards 


Fig.  16. — Showing  the  cracking  and 
scaling  due  to  decorticosis  (shell  bark) 
on  old  lemon  tree  trunks  in  California. 
Note  the  freedom  from  cracking  on 
the  orange  stock  below  the  bud  union. 


Bull.  395]       BARK   DISEASES   OF    CITRUS   TREES   IN    CALIFORNIA  49 

where  observations  have  been  made  for  about  12  years,  the  shell 
bark  has  reappeared  in  a  more  or  less  active  state  in  cycles  of  about 
four  or  five  years. 

The  general  effect  on  the  foliage  and  appearance  of  the  tree  may 
not  show  any  striking  damage  from  its  presence.  The  bark  under- 
neath the  affected  areas  has  time  to  recover  almost  as  rapidly  as  the 
disease  advances,  so  that  no  striking  effect  is  noted. 

Contributing  Conditions. — Shell  bark  is  apparently  more  preva- 
lent in  the  moister  coastal  and  foothill  sections  than  in  the  dryer  inte- 
rior valleys.  Degree  of  moisture  in  the  atmosphere  is  probably  one 
contributing  condition.  The  disease  is  also  likely  to  start  in  slight 
injuries  or  growth  cracks  in  the  outer  bark  tissues.  The  most  com- 
mon place  for  the  diseased  areas  to  start  is  the  bud  union  probably  at 
injuries  or  growth  cracks.  If  the  moist  soil  is  allowed  to  remain 
against  the  bark  at  or  above  the  bud  union  a  favorable  condition 
exists  for  the  start  and  development  of  the  disease. 

Susceptibility  of  varieties  and  strains  is  also  a  contributing  condi- 
tion. Observations  regarding  this  disease  indicate  that  Eureka  lemons 
are  more  susceptible  than  Lisbon  and  that  certain  strains  within  the 
Eurekas  are  more  resistant  than  other  strains  of  the  same  variety. 
In  general  Eureka  trees  are  15  to  20  years  old  before  shell  bark  begins 
to  affect  the  bark  to  any  extent.  Lisbon  lemon  trees  are  usually  older 
than  this  before  they  begin  to  show  its  effects  and  even  then  its  prog- 
ress on  Lisbon  is  usually  very  slow. 

Examples  of  extremely  susceptible  strains  of  Eurekas  have  been 
noted  on  which  shell  bark  began  at  the  age  of  8  to  10  years.  When 
the  parent  trees  from  which  these  susceptible  ones  had  been  budded 
were  examined,  it  was  found  that  the  trunks  and  also  the  bases  of  the 
oldest  limbs  were  covered  with  shell  bark.  There  were  indications  on 
the  trunks  that  a  new  bark  had  formed,  two  or  three  different  times, 
by  a  repetition  of  the  shelling  off  process.  Trees  from  the  same  source 
which  had  been  sent  to  another  locality  many  miles  distant  also 
showed  this  tendency  to  contract  shell  bark  at  an  early  age. 

A  second  example  is  that  found  in  an  orchard  where  12  trees,  18 
years  of  age,  at  the  end  of  a  single  row,  had  shell  bark,  while  no  trees 
on  adjoining  rows  had  contracted  the  disease.  Further  observation 
showed  that  these  12  trees  were  of  one  type  and  probably  came  from 
the  same  bud  stick.  It  was  later  learned  that  the  trees  had  been  bud- 
ded with  lemons  after  they  had  been  planted  and  that  the  budding 
had  probably  proceeded  in  the  direction  indicated  by  the  row  of 
affected  trees. 


50  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Control. — The  control  methods  for  this  disease  are  still  in  an  ex- 
perimental stage  in  California.  Certain  suggestions  as  to  prevention 
and  treatment  however  arise  from  the  knowledge  of  the  nature  of 
the  disease  and  observations  regarding  the  contributing  factors. 

Prevention.— -As  in  many  other  bark  diseases  the  avoidance  of 
injuries  to  the  bark  is  important.  It  is  expected  also  that  certain  dis- 
infectants or  fungicides  will  be  a  help  in  preventing  the  development 
of  the  organism.  The  selection  of  resistant  varieties  or  strains  is  also 
a  possible  means  of  preventing  the  disease  in  new  plantings. 

Observations  indicate  the  importance  of  selecting  buds  from  trees 
that  are  free  from  shell  bark  after  they  have  attained  a  considerable 
age.  Such  buds  will  probably  be  more  likely  to  give  trees  that  are 
resistant  to  shell  bark  than  trees  from  bud-wood  taken  from  a  tree, 
under  the  same  conditions,  that  has  already  contracted  the  disease. 

Treatment. — Whether  it  is  feasible  to  treat  trees  for  shell  bark 
depends  largely  on  the  age  of  the  trees  and  their  degree  of  suscepti- 
bility. 

When  the  disease  is  just  starting  on  a  tree  and  appears  to  be  pro- 
gressing rapidly,  results  so  far  secured  seems  to  indicate  that  it  is  a 
good  practice  to  scrape  not  only  the  diseased  area  (much  in  the  same 
manner  as  is  done  in  the  treatment  of  psorosis  of  oranges),  but  also 
to  scrape  the  entire  trunk,  being  careful  to  remove  only  a  thin  outer 
layer  so  as  to  disturb  as  little  as  possible  the  inner  bark  and  cambium. 
The  trunks  can  be  treated  with  some  good  fungicide  such  as  is  used 
in  psorosis  treatment  (page  44).  It  seems  advisable  also  to  thin  out 
the  branches  in  the  top  at  the  same  time  that  this  treatment  is  given. 

When  shell  bark  has  already  involved  the  entire  trunk,  or  a 
greater  portion  of  it,  and  a  new  bark  has  been  formed  underneath 
and  the  old  is  shelling  off,  it  is  uncertain  whether  a  severe  scrap- 
ing is  then  advisable,  especially  on  trees  where  the  disease  is  progress- 
ing slowly  and  is  not'  complicated  with  gum  diseases  such  as  Botrytis 
gummosis  or  other  forms  of  gum  disease.  These  gum  diseases  often 
follow  or  accompany  the  shell  bark  and  should,  of  course,  be  treated. 

A  thinning  of  the  branches  appears  to  be  beneficial  in  trees  that 
have  deteriorated  on  account  of  the  presence  of  shell  bark. 


Bull.  395]       BARK   DISEASES   OF    CITRUS   TREES   IN    CALIFORNIA 


51 


DIPLODIA  GUMMOSIS57 

In  a  former  publication  r,s  it  was  stated  that  this  form  of  gumming 
had  not  been  seen  in  California,  but  was  common  in  southern  Florida 
and  Cuba.  Since  then  a  Diplodia  fungus  similar  to  that  of  Florida 
has  been  found  frequently  in  California,  especially  in  San  Diego 
County,  associated  with  a  gumming  of  large  branches.     A  gummosis 


Fig.  17. — Gumming  (at  the  point  of  the  arrow)  on  lemon  bark,  due  to 
Diplodia  sp.  following  a  severe  frost  injury. 

57  Fawcett,  H.  S.,  Diplodia  natalensis  as  a  gum-inducing  and  fruit-rotting 
fungus.  Keport  of  plant  pathologist,  Fla.  Agr.  Exp.  Sta.  Ann.  Report,  1911, 
01-67,1912. 

Gumming,  Report  of  the  former  Plant  Pathologist,  Fla.  Agr.  Exp.  Sta.  Ann. 
Report,  1912,  77-92,  1913. 

»8  Fawcett,  H.  S.,  Citrus  diseases  of  Florida  and  Cuba  compared  with  those 
of  California.  Univ.  of  Calif.  Agr.  Exp.  Sta.  Bull.  262:210,  1915. 


52  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

due  to  a  Diplodia  has  been  found  recently  in  the  Philippines  also.59 
This  fungus  appears  to  be  especially  active  in  California  in  connection 
with  the  so-called  "heart  rot"  induced  by  severe  freezes.  In  1913 
this  fungus  was  found  advancing  into  the  yet  unkilled  wood  of  large 
branches  which  had  been  cut  off  after  the  freeze  with  the  result  of 
frequent  gum  formation.  The  fungus  advanced  much  more  rapidly 
in  branches  whose  cut  ends  were  sealed  with  grafting  wax  than  in 
those  not  so  sealed.  The  Diplodia  attack  resulted  indirectly  in  the 
death  of  areas  of  bark,  but  the  fungus  advanced  much  more  rapidly 
in  the  woody  tissue  (fig.  17). 

Control. — The  heart  rot  due  to  Diplodia  following  a  severe  freeze 
is  difficult  to  control.  A  non  air-tight  disinfectant,  such  as  Bordeaux 
paste,  mercuric  cyanide  (1  part  in  1000  parts  of  25. per  cent  denatured 
alcohol)  should  be  used  to  disinfect  the  tools  and  cut  surfaces.  This 
treatment  may  be  followed  by  a  sealed  covering  sometime  later  when 
the  wound  has  thoroughly  dried  out.  If  only  one  application  is  to  be 
used,  a  thin  substance  like  the  higher  boiling  coal-tar  products,  such  as 
Avenarius  carbolineum  or  Arrow  carbolineum,  may  be  used.  All  parts 
of  the  tree  cut  back  should  be  thoroughly  whitewashed  to  prevent  sun- 
burning. 

DOTHIORELLA  GUMMOSIS 

A  form  of  gummosis  similar  to  Diplodia  gummosis  on  lemon  trees 
in  California  has  been  shown  by  recent  experiments  to  be  due  to  a 
fungus  similar  to  Dothiorella  ribes  Gross  and  Dug.  (Botryosphaeria 
ribes).*  Gum  in  considerable  quantities  oozes  from  a  fusoid  gum 
pocket  on  the  trunk  or  large  limbs.  This  form  has  been  referred  to 
by  some  growers  as  "pocket  gum."  The  dead  bark  areas  vary  in  size 
but  are  usually  not  extensive.  The  inner  bark  tissue  is  often  disin- 
tegrated and  dissolved.  This  leaves  elongated  cavities  and  grooves  in 
the  inner  bark  next  to  the  cambium  region.  The  inner  bark  is  choco- 
late brown.  The  wood  below  the  pocket  is  often  discolored  inward  for 
a  short  distance  beyond  the  cambium.  There  is  also  a  gummous  zone 
near  the  cambium  like  that  described  in  Botrytis  and  Pythiacystis 
gummosis.  These  lesions  have  a  tendency  to  become  self  limited 
before  becoming  extensive  and  to  fill  up  with  new  bark  tissue  at  cer- 
tain seasons  and  later  to  become  active  again.  The  same  fungus  that 
causes  Dothiorella  gummosis  is  also  responsible  for  Dothiorella  rot  of 
fruit.  The  contributing  factors  and  control  would  probably  be  the 
same  as  those  given  for  Diplodia  gummosis. 


*  The  author  is  indebted  to  Dr.  C.  L.  Shear  for  identification  of  this  fungus. 
ssReinking,  O.  A.,  Philippine  Agriculturist,  9:123-127,  1921. 


BULL.  395]       BARK   DISEASES   OF    CITRUS   TREES   IN    CALIFORNIA 


53 


CITRUS  BLAST 

Citrus  blast  is  an  effect  of  a  bacter- 
ium, Pseudomonas  citriputeale,  on 
leaves  and  twigs.60  The  same  organ- 
ism also  produces  a  spot  on  fruit 
known  as  black  pit.61  For  detailed 
discussion  of  recent  investigations  on 
this  disease  see  account  by  Fawcett, 
Home  and  Camp.62 

Symptoms. — The  most  striking 
characteristic  of  the  blast  manifesta- 
tion is  an  area  shading  from  brown  to 
black,  which  usually  starts  in  a  tear 
or  break  of  the  wing  of  the  petiole 
and  extends  rapidly  into  the  base  of 
the  leaf  blade  and  to  the  twig  sur- 
rounding the  base  of  the  petiole.  Dur- 
ing cool,  damp  weather  these  areas 
enlarge  rapidly,  but  when  the  dry  and 
warmer  weather  comes  the  progress  of 
the  lesions  is  checked.  The  following 
features  are  to  be  noted :  there  is  a 
definite  line  between  the  dead  and 
live  tissue;  new  callus  tissue  forms 
under  the  affected  areas;  the  surface 
of  the  affected  bark  becomes  dry,  the 
initial  black  color  changes  to  a  reddish 
brown  and  scabs  are  formed.  If  the 
disease  progresses  very  rapidly  the 
leaves  wither  and  die  while  still  at- 
tached to  the  tree  (fig.  18)  ;  but  if  the 
progress  is  slow  or  if  it  occurs  at  a 
somewhat    warmer    temperature,    an 


G0  Lee,  H.  Atherton.  A  new  bacterial 
disease  of  Citrus.  Jour.  Agri.  Res.  3:108, 
1917. 

si  Smith,  C.  O.  Black  pit  of  lemon. 
Phyto.  3:277-283,  1913. 

62  Fawcett,  Home  and  Camp.  Citrus 
blast  and  black  pit.  Univ.  of  Calif.  Agri. 
Exp.  Sta.     Technical  paper,  5:1-24,  1923. 


Fig.  18. — Showing  small  lesions 
surrounding  attached  petioles. 
These  have  attained  their  maxi- 
mum development  at  the  end  of 
the  active  period.  Near  the  center 
is  a  typical  example  of  the  many 
dead  dry  leaves  which  frequently 
remain  firmly  attached  through 
the  subsequent  summer  and  fall. 


54  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

abscission  layer  is  formed  and  the  leaves  are  shed.  In  the  latter  case, 
it  seldom  progresses  further  than  the  petiole  of  the  leaf,  and  few 
lesions  will  be  formed  on  the  branches.  In  very  severe  cases  the 
disease  may  progress, so  rapidly  as  to  girdle  the  twigs  and  cause  many 
of  them  to  wither  and  die  -but  more  frequently  the  lesions  are  checked 
before  girdling  takes  place.  Microscopic  examinations  have  shown 
that  the  bacteria  are  most  abundant  in  the  layers  not  far  from  the 
cambium.  They  appear  to  attack  the  parenchyma  tissue  principally, 
and  do  not  kill  the  woody  tissue  except  in  extreme  cases.  When  the 
temperature  is  medium  or  high  after  the  disease  starts  some  gum 
formation  may  take  place.  As  black  pit  the  disease  manifests  itself 
on  the  fruit,  especially  in  the  case  of  lemons,  as  sunken  black  spots. 
As  these  develop  they  change  from  a  light  brown  to  dark  brown,  and 
then  to  black.  The  inner  white  part  of  the  peel  is  also  affected.  There 
is  a  collapse  of  the  tissue,  which  finally  becomes  light  brown  tinging 
to  reddish  brown  in  color. 

Control. — Spraying  experiments  have  been  carried  on  for  five 
consecutive  years,  and  the  results  each  year  have  indicated  that  Bor- 
deaux mixture,  if  applied  as  early  as  the  first  of  November,  will  pre- 
vent a  considerable  part  of  the  injury  from  this  disease.  It  is  of  little 
advantage  if  applied  some  weeks  later.  Since  seasons  favorable  to 
Citrus  blast  however  occur  only  about  once  in  three  years  it  appears 
doubtful  whether  the  expense  of  a  yearly  spraying  is  justified  by  the 
returns. 

It  is  believed  that  the  same  expense  in  good  cultural  care  will 
bring  greater  returns.  Observations  have  indicated  that  orchards 
which  are  well  taken  care  of  and  given  adequate  fertilization,  irriga- 
tion, and  other  cultural  attention,  are  usually  little  affected  as  com- 
pared with  orchards  where  it  has  been  impossible  to  give  all  the  care 
that  is  necessary.  Blast  appears  to  be  no  more  serious  than  a  light 
or  moderate  pruning  or  defoliation  by  dry  winds.  Orchards  protected 
from  the  driving  winds  that  often  come  with  the  rains  are  also  apt  to 
have  only  a  slight  amount  of  the  blast.  Windbreaks  or  other  protection 
from  the  wind  should  be  provided,  especially  for  new  plantings. 
Such  protection  together  with  proper  cultural  care  appears  to  prevent 
most  of  the  loss  from  Citrus  blast. 

These  suggestions  are  of  little  importance  for  conditions  in  south- 
ern California,  since  the  disease  is  a  minor  one  there,  usually  mani- 
festing itself  only  in  black  pit  of  the  fruit  which  is  seldom  of  sufficient 
importance  to  demand  a  remedy.  With  regard  to  protection  against 
injuries  the  same  principles  apply  to  all  sections. 


Bull.  395 J       BARK    DISEASES   OF    CITRUS   TREES   IN    CALIFORNIA  55 


TWIG  GUMMING 

A  gumming  and  dying  of  a  few  scattered  twigs,  especially  on  navel 
orange  trees,  in  the  late  summer  or  fall,  is  usually  of  minor  importance. 
It  occurs  in  both  California  and  Arizona.  The  cause  is  unknown.  It 
is  characterized  by  sudden  wilting  of  leaves  and  dying  back  of  twigs 
or  small  branches  to  a  distance  of  12  to  24  inches  from  their  tips,  At 
the  base  of  the  dead  portion,  the  bark  splits  and  gum  oozes  out  in 
considerable  quantities.  It  appears  to  occur  most  often  after  periods 
of  hot  dry  weather  and  has  been  known  in  California  for  many  years 
as  a  minor  trouble.  It  is  quite  distinct  from  Citrus  blast  though  some- 
times confused  with  this  disease. 

A  somewhat  similar  gumming,  associated  with  the  dying  back  of 
twigs  and  branches  but  without  the  characteristic  splitting  of  the  bark, 
has  frequently  been  noted  in  large  nursery  trees.  Although  certain 
organisms  have  been  found  in  the  lesions  none  of  them  have  reproduced 
the  disease  on  inoculation.  These  forms  of  twig  gummng  differ  from 
twig  blight  caused  by  the  cottony  rot  fungus,  Sderotinia  libertiana, 
in  the  absence  of  a  whitening  or  shredding  of  the  bark  and  of  the 
black  sclerotia  frequently  formed  in  the  bark  attacked  by  that  fungus. 


EXANTHEMA 

Nature  and  Symptoms. — Exanthema,  or  die-back  as  it  is  commonly 
called  in  Florida,  is  often  classed  as  a  gum  disease  although  gum  for- 
mation is  not  always  a  conspicuous  feature.  The  clear  gum  seen  in 
connection  with  exanthema  exudes  only  from  gum  pockets  on  the 
twigs,  or  is  formed  internally  near  the  center  of  the  fruit  at  the 
angles  of  the  segments  (fig.  19.)  Dark  excrescences  and  multiple  buds 
on  the  branches,  dying  back  of  terminal  branches,  compact  shortened 
growth  and  dark  irregular  reddish  brown  patches  on  the  surface  of 
the  fruit  are  other  symptoms. 

The  disease  has  been  considered  to  be  the  result  of  a  nutritional 
disturbance,  but  the  actual  cause  is  unknown.  Localized  areas  in 
California  orchards  showing  exanthema  have  frequently  been  noted 
where  sheep  corrals  or  cattle  barns  had  been  located  in  previous  years. 
It  has  also  been  found  in  other  instances  to  correspond  to  spots  where 
the  top  soil  had  been  graded  off  in  leveling  the  orchard  before  planting. 
Although  the  use  of  large  amounts  of  organic  nitrogenous  fertilizers, 


56 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


such  as  dried  blood  and  cottonseed  meal  have  been  considered  to  be 
contributing"  factors  in  inducing  exanthema  on  some  soils  in  Florida, 
these  fertilizers  have  not  been  observed  as  a  rule  to  encourage  the 
disease  under  conditions  in  California, 


Fig.  19. — Cross-section  of  a  green  orange  from  a  tree  affected  with  exan- 
thema. The  dark  areas  between  the  septa  next  to  the  core  indicate  the 
presence  of  clear  gum  that  has  taken  the  place  of  the  normal  tissue. 


CONTROL 

In  many  cases  in  California  where  exanthema  has  been  trouble- 
some in  young  trees  in  localized  areas,  the  trees  have  outgrown  the 
trouble  without  any  special  treatment.  In  cases  where  the  contribut- 
ing conditions  are  such  as  may  be  economically  removed  by  the  grower 
the  remedy  is,  of  course,  obvious. 

Exanthema  is  of  such  minor  importance  in  most  sections  of  Cali- 
fornia that  no  study  has  yet  been  made  of  special  methods  of  control. 
The  methods  used  in  Florida  cannot  be  recommended  for  California 
without  trial  as  the  disease  is  here  manifested  under  such  widely  dif- 
ferent conditions. 


BULL.  395]       BARK   DISEASES   OP    CITRUS   TREES   IN    CALIFORNIA  57 


OTHER  MINOR  FORMS  OF  GUMMING 

In  addition  to  the  gum  diseases  already  discussed  there  frequently 
occur  minor  forms  of  gumming,  some  of  which  have  been  shown  to  be 
induced  by  microorganisms  while  other  forms  are  brought  about  by 
insect  injuries,  chemical  stimuli,  or  in  certain  cases  by  physical  effects 
of  the  environment. 

Most  of  these  forms  of  gumming  cannot  be  classed  as  definite 
diseases  and  are  of  minor  importance  commercially  as  compared  with 
the  preceding  diseases. 

PENICILLIUM  EOSEUM 

Gumming  due  to  Penicillium  roseum.  This  fungus,  which  forms 
small  pink  tufts  of  hyphae  and  spores  on  the  surface  of  lemon  bark, 
is  capable  of  inducing  a  small  amount  of  gum  exudation  and  death  of 
small  areas  of  bark,  as  has  been  shown  by  inoculations  with  pure 
cultures.  It  has  been  found  as  a  secondary  fungus  associated  with 
Botrytis  gummosis  and  shell  bark  on  lemon  trunks,  mostly  in  the 
moister  coastal  sections  of  California.  It  is  also  associated  with  a 
minor  decay  of  lemon  fruits. 

FUSARIUM 

Many  species  of  this  genus  of  fungus  produce  pinkish  to  red  pus- 
tules or  masses  of  spores  within  the  surface  layer  of  dead  bark  of 
lemon  trees.  In  general  appearance  and  color  these  red  masses  look 
to  the  unaided  eye  much  like  those  of  Penicillmm  roseum.  This  fungus 
also  has  been  shown  to  be  capable  of  inducing  the  formation  of  a  small 
amount  of  gum,  and  of  causing  a  very  limited  amount  of  injury  to 
bark  tissue  adjoining  a  wound  when  inserted  into  cuts  on  sound  tissue, 
but  it  did  not  produce  definite  disease  lesions.  It  has  frequently  been 
found  associated  with  brown  rot  gummosis  and  appears  to  be  capable 
of  increasing  the  severity  of  this  gummosis  when  associated  with 
Pythiacystis  citrophthora  (as  has  previously  been  mentioned  under 
" brown  rot  gummosis")  but  when  acting  alone,  Fusarium  appears  to 
be  of  minor  importance  in  gummosis.  It  should  be  stated,  however, 
that  Barrett03  has  found  species  of  Fusarium  constantly  associated 


es  Barrett,  J.  T.,  Dry  root  rot.     Calif.  Citrus  Inst.,  First  Ann.   Rept.   1919- 
1920,  p.  157. 


58  UNIVERSITY    OF    CALIFORNIA — EXPERIMENT   STATION 

with  the  'dry  root  rot'  of  Citrus  trees  in  connection  with  which  con- 
siderable gumming  is  frequently  noted.  Although  the  disease  has 
not  been  reproduced  by  inoculation,  this  fungus  is  thought  to  be  an 
important  factor  in  the  development  of  dry  root  rot. 

ALTERNARIA 

A  species  of  Alternaria  similar  to  Alternaria*  citri  has  very  fre- 
quently been  found  in  bark  tissue  associated  with  minor  forms  of 
gumming  on  lemon  trees.  Inoculation  with  pure  cultures  has  shown 
that  it  may  sometimes  induce  slight  gum  exudation  and  very  slight 
injury  to  tissue  adjacent  to  cuts  into  which  spores  were  inserted.  Small 
green  immature  navel  oranges  affected  with  black  rot  frequently  show 
gum  exudation  at  the  navel  cavity  apparently  due  to  the  presence  of 
Alternaria  citri. 

MISCELLANEOUS  ORGANISMS 

If,  during  the  season  for  Citrus  blast  and  black  pit  attack,  the 
weather  becomes  warmer  than  usual,  small  drops  of  gum  are  apt  to 
form  at  the  edges  of  the  twig  lesions  caused  by  the  Citrus  blast  bac- 
terium, Bacterium  citriputeale,  C.  0.  Smith.  Slight  gumming  has 
been  noted  also  on  cuttings  inoculated  with  this  bacterium  and  held 
in  moist  chambers  over  a  free  water  surface  at  constant  temperatures 
of  80°  and  93.4°F.  (30°  and  34°C),  but  not  at  temperatures  below 
this.  Control  punctures  produce  no  gum.  Large  light-green  lemon 
fruits  also  develop  gum  when  inoculated  with  the  same  organism  under 
the  same  conditions,  but  not  in  the  uninoculated  punctures. 

It  is  probable  that  various  other  fungi  are  responsible  for  minor 
forms  of  gumming  in  Citrus.  Observation  has  indicated  that  under 
certain  conditions  Armillaria  mellea,  Schizophyllus  commune  and 
other  wood  rotting  fungi  are  minor  agents  in  bringing  about  small 
gum  exudations, 

GUMMING   ASSOCIATED   WITH   INSECT   INJURIES 

The  injuries  made  by  a  number  of  insects  are  frequently  followed 
by  gumming,  usually  slight  in  amount.  Small  drops  of  gum  may  form 
on  fruit  at  injuries  produced  by  the  orange  tortrix  (Tortrix  citrana), 
and  on  small  tree  trunks  and  limbs  from  grasshopper,  katydid  and 
other  insect  injuries.  Gumming  has  frequently  been  noted  on  twigs 
badly  infested  with  the  California  red  scale.  To  what  extent  this 
gumming  may  be  due  to  secretions  of  the  insects  or  to  the  entrance 


Bull.  395]       BARK    DISEASES   OF    CITRUS   TREES   IN    CALIFORNIA  59 

of  microorganisms  at  the  time  of  injury  is  uncertain.  Our  negative 
results  from  mechanical  injuries  to  Citrus  kept  sterile  and  free  from 
chemical  stimuli  would  indicate  that  this  gumming  was  probably  not 
due  to  the  injury  or  wound  in  itself. 

GUMMING  ASSOCIATED  WITH  CHEMICAL  STIMULI 

Among  the  chemical  stimuli  that  have  been  seen  to  result  in  gum 
formation  occasionally  in  Citrus  orchards  may  be  mentioned:  (1) 
liquid  hydrocyanic  acid  spilled  on  the  soil  near  the  roots  of  trees;  (2) 
hydrocyanic  acid  gas  used  in  fumigation;  (3)  spray  mixtures  contain- 
ing copper  sulfate  not  properly  neutralized  with  lime  or  containing 
other  toxic  substances;  (4)  a  poison  containing  arsenic  in  contact  with 
the  bark. 

Hydrocyanic  acid  coming  in  contact  with  a  large  main  root  usually 
causes  the  death  of  a  strip  of  the  bark  on  the  trunk  and  even  on  the 
limbs  and  branches  leading  up  from  this  portion  of  the  trunk.  This 
frequently  results  in  considerable  gumming  adjacent  to  the  killed  strip 
and  is  sometimes  mistaken  for  a  definite  gum  disease.  Under  some 
conditions  the  shock  produced  by  fumigation  with  hydrocyanic  acid 
gas  appears  to  be  responsible  for  initiating  gumming  on  the  tree 
trunks  of  orange  trees  especially.  The  pressure  in  small  gum  pockets 
formed  near  the  cambium  produces  small  rifts  in  the  bark  through 
which  the  gum  exudes  in  small  drops,  sometimes  in  numerous  places 
on  the  trunk.  Sometimes  coils  or  strings  of  gum  will  be  pushed  out  of 
small  openings  one  to  two  weeks  after  fumigation.  Such  gumming 
is  usually  temporary  and  the  places  where  the  gum  has  formed  quickly 
close  up  and  fill  with  new  tissue  without  noticeable  injury  to  the  tree. 

Copper  sulfate  solution  spilled  on  the  soil  over  a  root  will  kill 
strips  of  bark  and  result  in  gumming  in  much  the  same  way  as  hydro- 
cyanic acid. 

Ant  poisons  containing  arsenic  when  allowed  to  leak  out  of  the 
containers  frequently  have  the  same  effect. 

PHYSICAL  EFFECTS  OF  THE  ENVIKONMENT 

Physical  effects  do  not  appear  to  be  as  a  rule  more  than  contribut- 
ing* conditions  for  Citrus  gumming  under  California  conditions,  other 
stimuli  acting  as  the  immediate  cause. 

Mechanical  Injuries. — I  have  not  been  able  to  induce  gum  exuda- 
tion on  healthy  Citrus  trees  by  mechanical  injuries  alone,  when  the 
injured  portions  were  kept  clean  and  reasonably  free  from  contami- 
nation with  microorganisms  or  unusual  chemical  substances.    The  fol- 


60  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

lowing  kinds  of  wounds  were  made:  vertical  and  horizontal  cuts 
through  the  bark ;  augur  holes  with  and  without  glass  or  wooden  plugs ; 
bruises  by  heavy  and  light  blows  of  blacksmith 's  hammer ;  long  hori- 
zontal and  vertical  slits  through  the  bark ;  areas  of  bark  cut  away,  etc. 
All  such  injuries  kept  clean,  healed  in  the  usual  way  without  gumming. 
When  purposely  infected  with  Botrytis  cinerea  or  other  injurious 
fungi,  however,  gumming  resulted. 

Burning  and  Freezing. — Sunburning  and  freezing  do  not  appear 
to  be  important  factors  in  initiating  gum  formation  directly,  but  they 
supply  the  conditions  for  the  entrance  of  Botrytis  cinerea,  Sclerotinia 
libertiana  or  other  fungi  which,  after  becoming  established  in  the 
injured  tissue,  may  advance  rapidly  and  induce  gumming  in  tissue 
apparently  sound.  Freezing  and  sunburning  often  get  credit  for 
causing  the  gumming  when  they  have  merely  opened  up  the  way  for 
the  real  causative  agencies. 


SUMMARY 

For  the  sake  of  brevity  this  summary  deals  with  average  conditions. 
Allowance  must  therefore  be  made  for  variations  in  treatment  for 
unusual  or  abnormal  conditions.  Consult  the  discussions  in  the  text 
for  more  detailed  statements, 

Pythiacystis  gummosis  and  mal  di  gomma. — Prevented  by  pulling 
the  soil  away  from  the  base  of  the  tree  trunk  until  the  tops  of  the  first 
main  roots  are  exposed;  by  keeping  the  soil  next  to  the  trunk  from 
becoming  excessively  wet;  by  avoiding  injuries  to  the  bark;  and  by 
painting  the  trunks  with  fungicide  such  as  Bordeaux  paste  or  wash, 
lime-sulfur  wash  or  a  safe  carbolineum.  Treated  when  not  too  far 
gone,  by  cutting  away  the  invaded  killed  bark,  but  not  necessarily  the 
outer  gummous  zone,  and  painting  the  wound  with  a  suitable  fungi- 
cide ;  by  scraping  away  any  outer  layers  of  dead  bark ;  by  painting 
exposed  portions  of  wood  (after  healing  of  edges  begins)  with  benzine- 
asphalt  paint  or  other  suitable  covering;  by  cutting  back  the  tops  on 
trees  severely  affected;  by  inarching  or  bridge  grafting  in  certain 
cases. 

Dry  Root  Rot. — Prevented  by  avoidance  of  excessive  water  and 
injuries ;  by  providing  good  drainage  and  aeration  to  the  roots.  Lesions 
not  too  far  advanced  treated  by  cutting  beyond  visible  margin  of  decay 
in  the  wood  and  bark  and  by  disinfecting  and  painting. 

Armillaria  Root  Rot. — Prevention  of  spreading  obtained  by  elimi- 
nating the  means  of  communication  along  the  roots  by  trenches  or 


Bull.  395]       BARK   DISEASES   OF    CITRUS   TREES   IN    CALIFORNIA  61 

other  methods.  Treated  if  not  too  far  along  as  in  dry  root  rot.  Fungus 
eliminated  from  soil  by  taking  out  all  old  roots  or  rotting  wood  and 
using  carbon  bisulphide. 

Botrytis  and  Sclerotinia  gummosis. — Prevented  by  the  same 
methods  as  to  soils,  water,  injuries,  fungicides  as  in  the  previous 
disease.  Treated  by  cutting  out  the  dead  bark  to  the  wood  and  by 
scraping  off  only  the  outer  bark  beyond  this,  where  the  inner  layer 
is  not  killed,  and  by  painting  with  fungicide  as  in  the  previous  disease. 

Psorosis  (scaly  bark)  of  orange  trees.- — Methods  of  prevention 
not  definitely  known.  Treated  by  scraping .  away  the  outer  affected 
bark  of  lesions  in  the  first  and  second  stages,  scraping  lightly  the 
bark  not  yet  visibly  affected  to  a  distance  of  6-8  inches  beyond  the 
lesion  in  each  direction  and  applying  Bordeaux  paste  or  other  suit- 
able fungicides ;  by  cutting  out  certain  badly  affected  limbs  altogether ; 
by  eliminating  certain  of  the  worst  trees  in  the  third  stage  of  the  dis- 
ease or  cutting  them  off  below  the  diseased  part  where  this  is  possible. 
Treatment  must  vary  much  according  to  the  stage  of  the  disease.  (See 
discussion  of  various  stages.) 

Decorticosis  (shell  bark)  of  lemons. — Probably  prevented  in  part 
by  avoiding  injuries  to  the  trunk  and  keeping  soil  away  from  bud 
union.  Treated,  if  very  active,  in  beginning  stages  by  scraping  outer 
layers  of  bark  over  entire  trunk  and  disinfecting  as  in  psorosis. 

Diplodia  and  Dothioreila  gummosis  and  twig  gumming. — Treated 
by  eliminating  the  parts  affected  and  by  treating  cuts  with  fungicide 
followed  by  paint  where  wounds  are  large. 

Citrus  blast. — Prevented  in  part  by  the  use  of  windbreaks  to  avoid 
injuries  to  the  foliage  and  twigs  in  winter  and  spring;  by  cultural 
methods  conducive  to  good  summer  growth  and  a  minimum  of 
untimely  fall  growth.  May  be  prevented  in  part  also  by  spraying  with 
Bordeaux  not  later  than  November  1st  to  15th,  a  practice  however  not 
yet  considered  to  be  economically  successful. 

Exanthema. — Little  is  known  as  to  its  real  cause,  and  no  one  defi- 
nite method  of  prevention  or  control  can  be  suggested  for  California 
conditions  except  to  eliminate  certain  supposed  contributing  conditions 
where  this  is  possible.     (See  previous  discussion.) 

Minor  forms  of  gumming. — Not  many  of  these  are  sufficiently 
important  to  require  special  attention  either  as  to  prevention  or  treat- 
ment. Where  they  are  due  to  organisms  most  of  the  same  principles 
apply  as  are  given  for  the  previous  diseases.  Some  forms  are  depend- 
ent upon  conditions  that  cannot  be  controlled  but  recovery  often  fol- 
lows a  change  in  the  contributing  conditions. 


STATION  PUBLICATIONS  AVAILABLE  FOE  FEEE  DISTRIBUTION 


No. 

253.   Irrigation   and   Soil   Conditions  in  the 
Sierra  Nevada  Foothills,  California. 

261.  Melaxuma    of    the    Walnut,    "Juglans 

regia." 

262.  Citrus   Diseases  of  Florida   and   Cuba 

Compared  with  Those  of  California. 

263.  Size  Grades  for  Ripe  Olives. 

268.   Growing  and  Grafting  Olive  Seedlings. 
273.   Preliminary  Report  on  Kearney  Vine- 
yard Experimental  Drain. 

275.  The  Cultivation  of  Belladonna  in  Cali- 

fornia. 

276.  The  Pomegranate. 

277.  Sudan  Grass 

278.  Grain  Sorghums. 

279.  Irrigation  of  Rice  in  California. 

280.  Irrigation  of  Alfalfa  in  the  Sacramento 

Valley. 
283.   The  Olive  Insects  of  California. 
285.  The  Milk  Goat  in  California. 
294.   Bean  Culture  in  California. 
304.  A   Study  of  the  Effects  of  Freezes  on 

Citrus   in   California. 
310.   Plum  Pollination. 

312.  Mariout  Barley. 

313.  Pruning  Young  Deciduous  Fruit  Trees. 
319.   Caprifigs  and  Caprification. 

324.  Storage  of  Perishable  Fruit  at  Freezing 

Temperatures. 

325.  Rice  Irrigation  Measurements  and  Ex- 

periments    in      Sacramento     Valley, 

1914-1919. 
328.  Prune  Growing  in  California. 
331.  Phylloxera-Resistant  Stocks. 

334.  Preliminary  Volume  Tables  for  Second- 

Growth  Redwood. 

335.  Cocoanut    Meal    as    a    Feed    for  Dairy 

Cows  and  Other  Livestock. 

339.  The  Relative  Cost  of  Making  Logs  from 

Small  and  Large  Timber. 

340.  Control  of  the  Pocket  Gopher  in  Cali- 

fornia. 

343.  Cheese  Pests  and  Their  Control. 

344.  Cold  Storage  as  an  Aid  to  the  Market- 

ing of  Plums. 

346.  Almond  Pollination. 

347.  The  Control  of  Red  Spiders  in  Decidu- 

ous Orchards. 

348.  Pruning  Young  Olive  Trees. 

349.  A     Study    of     Sidedraft    and    Tractor 

Hitches. 

350.  Agriculture  in  Cut-over  Redwood  Lands. 

352.  Further  Experiments  in  Plum  Pollina- 

tion. 

353.  Bovine  Infectious  Abortion. 

354.  Results  of  Rice  Experiments  in   1922. 
357.  A    Self-mixing    Dusting    Machine    for 

Applying      Dry       Insecticides       and 
Fungicides. 


BULLETINS 
No. 
358. 


359. 
361. 


362. 
363. 


365. 
366. 

367. 

368. 

369. 
370. 
371. 

372. 

373. 
374. 

375. 

376. 

377. 
379. 
380. 

381. 

382. 

383. 

384. 


385. 
386. 


387. 
388. 


389. 
390. 


391. 
392. 


Black  Measles,  "Water  Berries,  and 
Related  Vine  Troubles. 

Fruit  Beverage  Investigations. 

Preliminary  Yield  Tables  for  Second 
Growth  Redwood. 

Dust  and  the  Tractor  Engine. 

The  Pruning  of  Citrus  Trees  in  Cali- 
fornia. 

Fungicidal  Dusts  for  the  Control  of 
Bunt. 

Avocado   Culture   in   California. 

Turkish  Tobacco  Culture,  Curing  and 
Marketing. 

Methods  of  Harvesting  and  Irrigation 
in  Relation  to  Mouldy  Walnuts. 

Bacterial  Decomposition  of  Olives  dur- 
ing Pickling. 

Comparison  of  Woods  for  Butter  Boxes. 

Browning  of  Yellow  Newtown  Apples. 

The  Relative  Cost  of  Yarding  Small 
and  Large  Timber. 

The  Cost  of  Producing  Market  Milk  and 
Butterfat  on  246  California  Dairies. 

Pear  Pollination. 

A  Survey  of  Orchard  Practices  in  the 
Citrus  Industry  of  Southern  Cali- 
fornia. 

Results  of  Rice  Experiments  at  Cor- 
tena,    1923. 

Sun-Drying  and  Dehydration  of  Wal- 
nuts. 

The  Cold  Storage  of  Pears. 

Walnut  Culture  in  California. 

Growth  of  Eucalyptus  in  California 
Plantations. 

Growing  and  Handling  Asparagus 
Crowns. 

Pumping  for  Drainage  in  the  San 
Joaquin  Valley,   California. 

Monilia  Blossom  Blight  (Brown  Rot) 
of  Apricot. 

A  Study  of  the  Relative  Values  of  Cer- 
tain Succulent  Feeds  and  Alfalfa  Meal 
as  Sourses  of  Vitamin  A  for  Poultry. 

Pollination  of  the  Sweet  Cherry. 

Pruning  Bearing  Deciduous  Fruit 
Trees. 

Fig  Smut. 

The  Principles  and  Practice  of  Sun- 
drying  Fruit. 

Berseem  or  Egyptian  Clover. 

Harvesting  and  Packing  Grapes  in 
California. 

Machines  for  Coating  Seed  Wheat  with 
Copper   Carbonate   Dust. 

Fruit  Juice  Concentrates. 


CIRCULARS 

No.  No. 

87.   Alfalfa.  154. 
113.   Correspondence  Courses  in  Agriculture. 

117.  The    Selection    and    Cost    of    a    Small  155. 

Pumping   Plant.  157. 

127.   House  Fumigation.  160. 

129.   The  Control  of  Citrus  Insects.  164. 

136.  Slelilotus    indica    as    a    Green-Manure  165. 

Crop  for  California. 

144.    Oidium  or  Powdery  Mildew  of  the  Vine.  166. 

151.  Feeding  and  Management  of  Hogs.  167. 

152.  Some  Observations  on  the  Bulk  Hand-  170. 

ling  of  Grain  in  California. 


Irrigation   Practice   in   Growing   Small 

Fruit  in  California. 
Bovine  Tuberculosis. 
Control  of  the  Pear  Scab. 
Lettuce  Growing  in  California. 
Small  Fruit  Culture  in  California. 
Fundamentals   of   Sugar    Beet   Culture 

under  California  Conditions. 
The  County  Farm  Bureau. 
Feeding  Stuffs  of  Minor  Importance. 
Fertilizing  California  Soils  for  the  1918 

Crop. 


CIRCULARS—  (Continued) 


No. 

173.  The   Construction    of    the    Wood-Hoop 
Silo. 

178.  The  Packing  of  Apples  in  California. 

179.  Factors    of    Importance    in    Producing 

Milk  of  Low  Bacterial  Count. 
184.   A  Flock  of  Sheep  on  the  Farm. 
190.  Agriculture  Clubs  in   California. 
199.   Onion  Growing  in  California. 

202.  County   Organizations   for   Rural   Fire 

Control. 

203.  Peat  as  a  Manure  Substitute. 

209.  The  Function  of  the  Farm  Bureau. 

210.  Suggestions  to  the  Settler  in  California. 
212.   Salvaging  Rain-Damaged  Prunes. 

214.  Seed  Treatment  for  the  Prevention  of 

Cereal  Smuts. 

215.  Feeding  Dairy  Cows  in  California. 
217.  Methods   for   Marketing  Vegetables   in 

California. 
220.   Unfermented  Fruit  Juices. 
228.   Vineyard  Irrigation   in  Arid  Climates. 

230.  Testing   Milk,    Cream,    and    Skim   Milk 

for  Butterfat. 

231.  The  Home  Vineyard. 

232.  Harvesting    and    Handling    California 

Cherries  for  Eastern   Shipment. 

233.  Artificial  Incubation. 

234.  Winter  Injury  to  Young  Walnut  Trees 

during  1921-22. 

235.  Soil  Analysis  and  Soil  and  Plant  Inter- 
relations. 

236.  The  Common  Hawks  and  Owls  of  Cali- 

fornia   from    the    Standpoint    of    the 
Rancher. 

237.  Directions  for  the  Tanning  and  Dress- 

of  Furs. 

238.  The  Apricot  in   California. 

239.  Harvesting  and  Handling  Apricots  and 

Plums  for  Eastern  Shipment. 

240.  Harvesting    and    Handling    Pears    for 

Eastern    Shipment. 

241.  Harvesting  and  Handling  Peaches  for 

Eastern   Shipment. 

242.  Poultry   Feeding. 

243.  Marmalade  Juice  and  Jelly  Juice  from 

Citrus  Fruits. 

244.  Central  Wire  Bracing  for  Fruit  Trees. 

245.  Vine  Pruning  Systems. 

247.  Colonization  and  Rural  Development. 

248.  Some  Common  Errors  in  Vine  Pruning 

and  Their  Remedies. 

249.  Replacing  Missing  Vines. 

250.  Measurement   of    Irrigation    Water   on 

the  Farm. 

251.  Recommendations  Concerning  the  Com- 

mon     Diseases     and     Parasites     of 
Poultry  in   California. 

252.  Supports   for  Vines. 

253.  Vineyard  Plans. 

254.  The  Use  of  Artificial  Light  to  Increase 

Winter  Egg  Production. 

255.  Leguminous   Plants  as  Organic  Fertil- 

izer in   California  Agriculture. 


No. 

256. 

257. 

258. 

259. 

260. 

261. 
262. 
263. 
264. 

265. 
266. 

267. 

268. 

269. 
270. 
271. 
272. 

273. 

274. 

275. 

276. 

277. 

278. 
279. 
281. 


282. 

283. 
284. 
285. 
286. 
287. 
288. 
289. 
290. 
291. 

292 
293. 
294. 
295. 

296. 

297. 

298. 

299. 


The  Control  of  Wild  Morning  Glory. 

The  Small-Seeded  Horse  Bean. 

Thinning  Deciduous  Fruits. 

Pear  By-products. 

A  Selected  List  of  References  Relating 
to  Irrigation  in  California. 

Sewing  Grain   Sacks. 

Cabbage  Growing  in  California. 

Tomato  Production  in  California. 

Preliminary  Essentials  to  Bovine  Tuber- 
culosis Control. 

Plant  Disease  and  Pest  Control. 

Analyzing  the  Citrus  Orchard  by  Means 
of  Simple  Tree  Records. 

The  Tendency  of  Tractors  to  Rise  in 
Front;  Causes  and  Remedies. 

Inexpensive  Lavor-saving  Poultry  Ap- 
pliances. 

An  Orchard  Brush  Burner. 

A  Farm  Septic  Tank. 

Brooding  Chicks  Artificially. 

California  Farm  Tenancy  and  Methods 
of  Leasing. 

Saving  the  Gophered  Citrus  Tree. 

Fusarium  Wilt  of  Tomato  and  its_  Con- 
trol by  Means  of  Resistant  Varieties. 

Marketable  California  Decorative 
Greens. 

Home  Canning. 

Head,  Cane,  and  Cordon  Pruning  of 
Vines. 

Olive  Pickling  in  Mediterranean  Coun- 
tries. 

The  Preparation  and  Refining  of  Olive 
Oil  in  Southern  Europe. 

The  Results  of  a  Survey  to  Determine 
the  Cost  of  Producing  Beef  in  Cali- 
fornia. 

Prevention  of  Insect  Attack  on  Stored 
Grain. 

Fertilizing  Citrus  Trees  in  California. 

The  Almond   in   California. 

Sweet  Potato  Production  in  California. 

Milk    Houses    for    California    Dairies. 

Potato    Production    in    California. 

Phylloxera   Resistant   Vineyards. 

Oak  Fungus  in  Orchard  Trees. 

The  Tangier  Pea. 

Blackhead   and   Other   Causes   of  Loss 

of  Turkeys  in  California. 
Alkali  Soils. 

The  Basis  of  Grape  Standardization. 

Propagation   of  Deciduous  Fruits. 

The  Growing  and  Handling  of  Head 
Lettuce  in  California. 

Control  of  the  California  Ground 
Squirrel. 

A  Survey  of  Beekeeping  in  California  ; 
The   Honeybee   as   a   Pollinizer. 

The  Possibilities  and  Limitations  of 
Cooperative  Marketing. 

Poultry    Breeding   Records. 


The  publications  listed  above  may  be  had  by  addressing 

College  of  Agriculture, 

University  of  California, 

Berkeley,  California. 


15m-10,'25 


